Interference & Diffraction

$Page 1: Interference & Diffraction$
Interference & Diffraction12

COMPREHENSIVE REVIEW

1. The redistribution of intensity of the light wavesdue to the superposition of waves is calledinterference.

2. The interference was discovered by ThomasYoung in 1801.

3. Coherent and Non-coherent sources

Two sources of light are said to be coherentif their frequency is same and the phase differencebetween the superposing waves from them doesnot change with time. However, the frequencyof the waves from the noncoherent sources maybe different and the phase difference also maychange with time.

4. When the interference occurs with the coherentsources, amplitudes of the superposing waves addlike vectors.

That is, 1 2A A A

5. When the interference occurs with the non-coherent sources, the intensities are added likescalars.

That is : 1 2I I I

6. For interference between the light waves fromthe coherent sources, the phase difference ()between the light waves is the angle betweenthe amplitude vectors.

That is : 1 2A A A

And 2 2 21 2 1 2A A A 2A A cos

or 1 2 1 2I I I 2 I I cos

Note that 2I A7. Coherent sources can be created either by division

of amplitude or by division of wave front.

i) Examples of coherent sources by divisionof wave front :

Fresnel's biprism, Lloyd's mirror, Young'sdouble slit experiment.

INTERFERENCE AND DIFFRACTION ( 27 )

ii) Examples of coherent sources by divisionof amplitude.

Newton's rings, interference at thin films.

8. Interference can occur both between transverseand longitudinal waves.

9. In interference energy is NOT destroyed. On theother hand it is redistributed.

10. At constructive interference due to sources ofsame intensity, the intensity is more than that ofone source and at destructive interference it isless than that due to one source.

11. The maximum possible value of intensity isproportional to (A

1 + A

2)2 and the minimum

possible is proportional to (A1 – A

2)2.

12. At maximum the amplitude is A1 + A

2 and at

minimum the amplitude is A1 – A

2.

Also2

max 1 22

min 1 2

I (A A )

I (A A )

13. The phase difference between the superposingwaves for maximum should be :

2n

where n = 0, 1, 2, 3, .....

The phase difference between the superposingwaves for the minimum should be,

(2n 1)

where n = 0, 1, 2, 3, ......

14. Phase difference and path difference x arerelated to each other as follows :

2x

15. The path difference for maximum of the interferencepattern is :

x n

where n = 0, 1, 2, 3, ....

The path difference for the minimum of interferencepattern is given by :

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x (2n 1)2

where n = 0, 1, 2, 3, .....

16. Young's double slit experiment

It consists of a source S placed symmetricallywith respect to two slits S

1 and S

2. A screen is

placed in front of the slits.

Fig. 16.1

The fringe pattern is formed on the screen.The line SO is perpendicular to both the screenand plane passing through S

1 and S

2.

Distance between the slits is denoted by dand that between the slits and the screen isdenoted by D.

The bright fringe formed at O is calledcentral maximum.

a) The fringe width is same for all fringes andis given by :

D

d

b) The location of nth bright fringe on thescreen is given by :

n

Dy (b) n n

d

Here ny (b) is the distance of nth bright

fringe from O. And n = 0, 1, 2, 3, .....

c) The distance of nth dark fringe is given by :

n

Dy (d) (2n 1) (2n 1)

2 2 d

Here n* = 1, 2, 3, .....

d) The angular separation** for nth bright fringeis given by :

n

n(b) n

D d

where n = 0, 1, 2, 3, .....

e) The angular separation for nth dark fringe isgiven by :

n (d) (2n 1)2d

17. If one of the slit is closed. then the distribution ofthe intensity of light both for coherent as well asnon-coherent sources is same.

18. Path difference on reflection. Stoke's law

When a wave of light is reflected from thesurface of a denser medium. the phase of thereflected ray changes by . That is. an additional

path difference equal to 2

is introduced for the

reflected ray.

19. Interference at thin films

i) By reflection :

Path difference 2 t cos r

ii) By transmission :

Path difference 2 t cos r

where = refractive index, t = thickness ofthe film and r is angle of refraction.

iii) If at any point on the thin film we obtainmaximum by reflection. Then at the samepoint we obtain minimum by transmission.Vice versa is also true. That is the interferencepatterns by reflection and transmissions arecomplimentary.

iv) In case the film is of negligible thicknessi.e. t < then net path difference in the

reflected pattern will be less than 2

and

hence the film will appear dark. In otherwords no reflection will appear to beoccurring from the film.

v) In case of t << , negligible path differencewill be introduced by the film, hence the filmappears to be bright.

vi) Due to the difference in the amplitudes ofthe waves by reflection at the surface andthose by refraction from the film, the minimaare not distinct. It is found that maximumdistinctness is obtained when the angle ofincidence is 450.





viii) The thickness of the non-reflecting film is

t ,4

where is the length of the wave

in the film and is the refractive index ofthe film.

20. Interference at wedge shaped film

Wedge shaped air film can be obtained byplacing a thin wire between the plates.

Let d = diameter of the wire, then the fringeband width is given by :

x

2d

where x = distance from the edge of the wedgewhere wire is placed.

21. Newton's rings by reflection

i) Diameter of the nth dark ring is given by,

1/ 2

n

4nRD (d)

where R = radius of curvature of theconvex surface of the plano-convex lens and = refractive index of the medium betweenthe lens and plane plate.

ii) Diameter of nth bright ring is,

1/ 2

n

2(2n 1)RD (b)

22. Newton's rings by transmission

i) Diameter of nth bright ring is,

1/ 2

n

4nRD (b)

ii) Diameter of nth dark ring is,

1/ 2

n

2(2n 1)RD (d)

iii) The reflected and transmitted patterns arecomplementary. The central fringe is darkin the reflected pattern and bright in thetransmitted pattern.

23. If the angle of the biprism is increased thendistance between the sources increases and hencefringe width decreases.

24. If the slit width in the Young's experiment iscontinuously increased, then for very small valuesof the width the intensity increases, but for largevalues of width general illumination occurs.

25. The intensity of the interference maximum fromtwo sources of equal intensity is 4 I. The intensityof light at that point when one slit is closed is I.

26. Conditions for obtaining a sustained

interference.

a) Sources of light must be monochromatic andof the same frequency.

b) Two sources must be coherent.

c) Amplitudes of the two sources shouldpreferably be equal.

d) The distance between the two sources mustbe small.

e) The sources must be narrow.

f) The distance between the sources and thescreen should be large.

g) The interfering light waves must be in thesame states of polarization.

27. Fresnel's biprism experiment

a) In the Fresnel's biprism, the two coherentsources are obtained by refraction at a thinprism.

b) The biprism consists of two right angledprisms put together base to base and each

having refracting angle equal to 1

2 degree.

Fig. 27.1

c) The distance between the two coherentsources is given by :

d 2a( 1)

where, a = distance between the source andthe prism, is the refractive index of theprism with respect to the surroundingmedium, = refracting angle of each prismin radians.





d) The distance (D) between the coherentsources and the screen is given by :

D a b where b is the distance between the screenand the biprism.

e) The fringe width is given by :

(a b)

2a( 1)

f) If the experiment is repeated in a liquid ofrefractive index ', then the fringe width isgiven by :

(a b)(a b)'

'2a( ')

2a 1'

Hence, ' 1

'

Here ' 1 but . Hence ' . g) The fringe width in Fresnel's biprism

experiment in a liquid (1 ' ) increases,

whereas in case of Young's double slitexperiment it decreases.

28. The diffraction is the phenomenon of bending ofwaves around the corners of obstacles or theapertures. Due to the diffraction, the wavesencroach into the geometrical shadows of theobstacle or aperture.

29. The interference occurs due to the superpositionof wavelets from two wave fronts. But thediffraction occurs due to the superposition ofwavelets from the two parts of the same wavefront. Like interference, the diffraction is also acharacteristic property of all types of waves.

30. Huygens-Fresnel half period zone theory of

diffraction.

a) The wave front originating from the sourceand striking the aperture or obstacle isdivided into a number of circular andconcentric half period zones.

b) The centre of half period zones lies at thepoint of intersection of the wavefront andthe line joining the source and the observer.

c) The outer boundary of pth half period zone is

at a distance b p / 2 from the observation

point. Here b is the distance of the screenfrom the slit.

d) The boundaries of the circles bounding pth

half period zone lies at distance b p / 2

and b (p 1) / 2 from O.

e) Area of every half period zone is same andequal to b.

f) The radius of pth half period zone is,

pr pb

g) The light waves originating from theconsecutive half period zone reach O with

a path difference of .2

h) The amplitude of the light reaching O goeson decreasing as the serial number of halfperiod zones increases.

i) The secondary wavelets from the consecutivehalf period zones meet at O in the oppositephase.

j) If A1, A

2, A

3 ,..... A

n be the amplitudes of

the light waves reaching O from 1st, 2nd, 3rd,......, nth half period zone, then resultantamplitude of the wave at O will be :

1 2 3 4A A A A A ......

Here 1 2 3A A A ......

We can write : 1 32

A AA

2

1 33

A AA

2

and so on





Hence 1 nA AA ,

2 2 when n is odd

And 1 nA AA ,

2 2 when n is even.

k) Therefore O will be maximum if the totalnumber of half period zones is odd and itwill be a minimum when the total number ofhalf period zones is even.

31. Characteristics of Fresnel diffraction

a) Both the source of light as well as the screenor the observer are at finite distance fromthe obstacle or the aperature.

b) The incident as well diffracted wave frontsare either spherical or cylinderical.

c) The centre of the diffraction pattern maybe maximum or minimum depending onwhether the number of half period zone isodd or even.

d) No lenses are used.

e) The diffraction pattern is the image of theobstacle or the aperture.

f) Only one aperture or one obstacle producediffraction.

g) The amplitudes of the wavelets fromdifferent half period zones go on decreasingas the number of half period zones increases.

32. Fraunhoffer d" raction

a) Both the source and the screen are at infinitedistance (real or virtual) from the obstacleor aperture (The virtual infinite distance canbe obtained by using lenses).

b) The incident wave front and the diffractedwave front are plane.

c) The centre of the diffraction pattern isalways maximum. It is true both for thediffraction at an aperture or that at anobstacle.

[Poisson was the first to show thatthe centre of the shadow of an opaqueobject should be a bright spot. He thoughtit absurd. But, later it was found to be true.It occurs due to diffraction. However, theintensity of the bright spot is measurableonly when the size of the obstacle is veryvery small].

d) Lenses are used.

e) The diffraction pattern is the image of thesource.

f) The diffraction effect may be due to morethan one apertures and obstacles.

33. Theory of Fraunhoffer diffraction for aperture

a) The minimum occurs at a point on eitherside of the central maximum, such that thepath difference between the waves from thetwo ends of the aperture is given by :

= nwhere n = 1, 2, 3, .....

b) The secondary maximum occurs where thepath difference between the waves from thetwo ends of the aperture is given by :

(2n 1)2

where n = 1, 2, 3, .....

c) The intensity of the secondary maxima isless than the central maximum and goes ondecreasing as the order of maximumincreases.

d) The width of the central maximum is :

0

2 D

d

where D = distance between the apertureand the screen and d = width of the aperture.

e) The width of each secondary maximum issame and is given by :

D

d

That is : 0 2

Note. Compare the expression for withthe fringe width of interference pattern.

f) The angular width for the central maximumis :

00

2

D d

And that for each maximum is :

D d

That is : 0 2





g) The distance of nth secondary maximumfrom the mid point of central maximum isgiven by :

n

Dy (max) (2n 1)

2d

And the angle of diffraction for nth secondarymaximum is :

nn

y (max)(b) (2n 1)

D 2d

This is true only when n is small so that,

n nsin (b) (b)

h) The distance of nth minimum from the midpoint of central maximum is given by :

n

Dy (min) n

d

And the angle of diffraction is,

nn

y (min)(d) n

D d

This is also valid only when,

n nsin (d) (d)

i) In general, the angular location of nth secondarymaximum is given by :

nsin (b) (2n 1)2d

And that for the nth minimum is given by :

nsin (d) nd

That is : nd sin (d) n

34. Plane diffraction grating

The above relation is valid even for the planediffraction grating. However for the gratingd = a + b, where a = width of slit and b = widthopaque part.

Here d = a + b is called grating element.Hence, for plane grating, if be the angle ofdiffraction for nth minimum then :

(a + b) sin= n

If the diffraction pattern is focussed on thescreen with the half of a convex lens of focallength f, placed very near to the aperture, then

f D and hence in the various relations D maybe replaced by f.

35. Resolving power

a) The ability of an optical instruments to showtwo closely placed point objects separatelyis called resolving power.

b) Rayleigh's criterion for resolution

Two point objects can be seen separatewith the help of optical instrument, when thecentral maximum of the diffraction patternof one coincides with the first minimum ofthe diffraction pattern of the other.

c) Limit of resolution is defmed as the reciprocalof the resolving power.

d) Limit of resolution for eye and telescope isexpressed as the angle subtended by the twoobjects at the eye or the objective of thetelescope.

It is given by :

For eye :

1

60 part of degree = 1'

For telescope :

1.22

d

Here,

d = diameter of the aperture of the objective.

e) The limit of resolution for the microscope isdefined as the minimum distance betweenthe point objects so that they are seenseparately with the microscope. It is givenby :

0.61 a

sin

where,

= refractive index of the medium inwhich the object is placed and is thesemivertical angle of the cone of light fromthe object. Here sin is called numericalaperture of microscope.





S.No. Interference

1. Results due to the

superposition of waves

from two coherrent

sources.

2. All fringes are of same

width

D

d

3. All fringes are of same

intensity

4. Intensity of all

minimum may be zero.

5. Positions of nth

maxima and minima.

n

n Dy (b)

d

n

Dy (d) (2n 1)

d

6. Path difference for nth

maxima

n

7. Path difference for nth

minima

(2n 1)

Diffraction

Results due to the

superposition of

wavelets from different

parts of same wave

front. (single coherrent

source)

All secondary fringes

are of same width but

the central maximum is

of double the width

0

D2 2

d

Intensity decreases as

the order of maximum

increases.

Intensity of minima is

not zero.

Positions of nth

secondary maxima and

minima

n

Dy (b) (2n 1)

d

n

n Dy (d)

d

Path difference for nth

secondary maxima

(2n 1)2

Path difference for nth

minima

n

36. Comparison between interference & diffraction.





Interference of Light

1. In the biprism interference experiment, what isthe nature of the source of light ?

a) Narrow slit b) Extended

c) Point source d) None of the above

2. A very very thin (t << ) film is seen in whitelight. It will appear :

a) White b) Red

c) Violet d) Black

3. What is the nature of graph between fringe widthand the distance between the slits of Young'sdouble slit experiment ?

a) Straight line b) Parabola

c) Hyperbola d) Ellipse

4. In the Fresnel's biprism experiment, the twocoherent sources are obtained by :

a) reflection b) refraction

c) internal reflection d) two slits

5. Which of the following produces highly coherentbeam of light ?

a) Biprism

b) Lloyd's mirror

c) Laser

d) Michelson interferometer

6. In the Young's double slit experiment, the distanceof pth dark fringe from the central maximum is :

a) D

(2p 1)d

b)

D(2p 1)

d

c) D

(2p 1)2d

d)

D(2p 1)

2d

7. Who was the first to study the phenomenon ofinterference ?

a) Huygen b) Young

c) Fresnel d) Lloyd

8. What is the ratio of phase difference to the pathdifference of two light waves ?

a) 2 b) 2

c) 2

d) 1

2

MULTIPLE CHOICE QUESTIONS

9. In Young's double slit experiment with mono-chromatic light the central fringe will be :

a) Coloured b) White

c) Bright d) Black

10. Two sources of light are said to be coherent whenboth give out light waves of same :

a) amplitude and phase

b) wave length and constant phase difference

c) intensity and wavelength

d) phase and speed

11. For sustained interference we need two sourceswhich emit radiation :

a) with same intensity

b) with same amplitude

c) having constant phase difference

d) in a narrow beam

12. Which of the following is NOT essential for twosources of light in Young's double slit experimentto produce a sustained interference ?

a) Equal frequency b) Equal wavelength

c) Equal intensity

d) Constant phase relationship

13. For sustained and sharp interference pattern, thewave length of the interference waves should :

a) be same

b) be different and wide apart

c) be different but close together

d) not obey the above mentioned conditions

14. In Young's double slit experiment, a maximum isobtained when the path difference between theinterfering waves is :

[Given that n = 1, 2, 3, 4, .....]

a) n2

b) n

c) (2n 1)2

d) other than that given above

15. Which of the following is conserved when lightwaves interfere ?

a) Amplitude b) Intensity

c) Phase d) None of the above





16. In the formation of colours in thin films, the opticalpath difference between the interfering rays doesnot depend upon :

a) thickness of the film

b) wavelength of light

c) angle of incidence

d) width of the slit from which light is incident

17. In which of the following the interference is NOT

produced by the division of wave front ?

a) Young's double slit experiment

b) Fresnel Biprism

c) Lloyd's mirror

d) Colours of thin film

18. For observing interference in thin films with alight of wave length A the thickness of the film :

a) may be of any magnitude

b) should be much smaller than

c) should be of the order of

d) should be a few thousand times of

19. What causes changes in the colours of the thinsoap or oil films ?

Variation in the :

a) angle of incidence

b) angle of reflection

c) thickness of the film

d) none of the above

20. A thin film is viewed in white light. The colour ofthe film seen at a particular point depends upon :

a) width of the source

b) distance of the source

c) location of the observer


21. In Young's double slit experiment, a minimum isobtained when the phase difference of thesuperimposing waves is :

[Given that n = 1, 2, 3, 4, .....]

a) zero b) n

c) (n + 1) d) (2n + 1)

22. S is the size of the slit, d is the separation betweenthe slits and D is the distance where Young'sdouble slit interference pattern is being observed.If be the wavelength of light, then for sharpfringes, the essential condition is :

a) S

D d

b)

S

D d

c) S dD d) SD d

23. If Young's double slit experiment is performedwith white light, the fringe are :

a) coloured b) white

c) white spot surrounded by coloured fringes

d) coloured fringes surrounded by white rings

24. The fringe width of interference fringes for redcolour is . The fringe width for the violet colourwill be nearly :

a) same b) double

c) four times d) eight times

25. In the Young's double slit experiment, the intensityof central maximum is I. If either of the slit isclosed, the intensity of the same location is I

0,

What is the relation between I and I0

?

a) 0I 4 I b) 0I 2 I

c) 0II

2 d) 0I

I4

26. The intensity of maximum in the Young's doubleslit experiment is I and that for the minimum iszero. What will be the intensity at a point where

the path difference is 5

?4

a) zero b) I

2

c) 2 I d) I

4

27. What is the colour of the interference fringenearest to the white central maximum in casethe source of light is white ?

a) Yellow b) Red

c) Blue d) Violet

28. If the Fresnel's biprism experiment is performedin water instead of air, then the distance (d)between the coherent sources becomes nearly :

a) 1.3 times b) 1.5 times

c) 2.5 times d) 3.3 times

29. The intensity of each coherent source is I0, Which

of the following gives the intensity at a pointwhere the phase difference between the superposing

waves is ?





a) 0I (1 cos b) 202 I cos

2

c) 02 I ( cos ) d) 204 I cos

2

30. In the Fresnel's biprism experiment, why therefracting angle is taken as very very small ?

a) To increase

b) To decrease

c) To increase intensity

d) To decrease intensity

31. The intensity of the light emerging from the twoslits in Young's experiment is in the ratio 1 to 4.The intensity of the minimum to the consecutivemaximum will be in the ratio of :

a) 1 : 2 b) 1 : 4

c) 1 : 9 d) none of the above

32. The amplitude of the light waves emerging fromthe two slits in Young's experiment is in the ratioof 2 : 3. The intensity of the minimum to that ofthe consecutive maximum will be in the ratio of :

a) 2 : 3 b) 4 : 9

c) 1 : 9 d) none of the above

33. The light waves from two coherent sources ofsame intensity 'I' interfere. At the minimum, theintensity of light is zero. What is the intensity oflight at the maximum ?

a) I b) 4 I

c) I2 d) 4 I2

34. Instead of using two slits as in Young's experiment,if we use two separate but identical sodium lamps,which of the following will occur ?

a) General illumination

b) Widely separate interference

c) Very bright maximum

d) Very dark minimum

35. The interference fringes formed by a thin oil filmon water, when seen in the yellow light of sodiumlamp will be :

a) coloured

b) yellow and black

c) coloured but without yellow colour

d) partly green and partly orange

36. For best contrast between maxima and minimain the interference pattern of Young's double slitexperiment, the intensity of light emerging out ofthe two slits should be :

a) equal b) double

c) small d) large

37. What happens to the fringe pattern when theYoung's double slit experiment is performed inwater instead of air ?

a) Shrinks b) Disappears

c) Unchanged d) Enlarged

38. How will the interference pattern in Young'sdouble slit experiment be affected if yellow lightis replaced by blue light ?

The fringes will be :

a) wider b) narrower

c) brighter d) fainter

39. In the double slit experiment the fringe width withlight of wavelength is 0.150 mm. What is theminimum width of each slit so that the interferencepattern is visible ?

a) 0.150 mm b) 0.150

mm

c) 0.150 mm d) 1

0.150

mm

40. In a certain region of a thin film we get 6 fringeswith light of wavelength 500 nm. How manyfringes will we get in same region with wave-length 600 nm ?

a) 6 b) 5

c) 30 d) 36

41. In a thin air film 8 fringes occur between twopoints. The light is incident normally on the film.The wavelength of light is 500 nm. What is thedifference in thickness of the film between twopoints ?

a) 2 m b) 2 mm

c) 2 cm d) none of the above

42. In the Young's experiment with sodium light, theslits are 0.589 m apart. What is the angular widthof the third maximum ?

[Given that, = 589 nm]

a) sin–1 (3 10–6) b) sin–1 (3 10–8)

c) sin–1 (0.33 10–6) d) sin–1 (0.33 10–8)





43. A slit illuminated by monochromatic light is held0.4 mm from the surface of a plane mirror. Thefringe width in the interference pattern obtainedwith reflected and direct light is 0.25 mm. Thescreen is at 50 cm from the slit. what is thewavelength of light ?

a) 400 nm b) 450 nm

c) 500 nm d) 550 nm

44. In a biprism experiment the distance of the secondmaximum was 2.0 mm on a screen 1 m awayfrom 0.5 mm apart slits. What is the wave lengthof light ?

a) 400 nm b) 450 nm

c) 500 nm d) 500 nm

45. Interference is produced with two coherent sourcesof same intensity. If one of the source is coveredwith a thin film so as to reduce the intensity oflight coming out of it to half, what will happen tothe interference pattern ?

a) Bright fringes will be less bright and darkfringes will be less dark

b) Bright fringes will be more bright and the darkfringes will be more dark

c) Brightness of both types of the fringes will besame (General illumination)

d) None of the above will occur

46. What is the nature of graph between the separationof the slits and fringe width in case of the Young'sdouble slit experiment ?

a) Straight line with –VE slope

b) Straight line with +VE slope

c) Rectangular hyperbola

d) Parabola

47. In Young's double slit experiment the slits are ofdifferent lengths and widths. The amplitude ofthe light waves is directly proportional to the :

a) length of the slit

b) distance between the slits

c) area of the slit d) width of the slit

48. In the Young's double slit experiment both the slitsare similar. If the length of one of the slits is halved,which of the following is true ?

a) Dark fringes become darker

b) Bright fringes become brighter

c) Dark fringes become brighter

d) Bright fringes become narrower

49. In order that a thin oil film on the surface of watermay show the changes in colour, its thicknessshould be of the order of :

a) 500 nm b) 50 nm

c) 500 m d) 50 m

50. During interference of light, energy is :

a) created at the maxima

b) destroyed at the minima

c) not conserved

d) redistributed

51. In the Young's double slit experiment, the separationbetween the slits is halved and the distancebetween the slits and the screen is doubled, thefringe width will be :

a) halved b) unchanged

c) doubled d) quadrupled

52. What should be the order of the thickness of oilfilm on the water surface to enable one see thecolour in it ?

a) 1 nm b) 1 m

c) 1 mm d) 1 cm

53. In the interference pattern produced by twoidentical slits, the intensity of central maximum isI. What will be the intensity of light at the samespot, if one of the slits is closed ?

a) I b) I / 2

c) I

4d)

I

8

54. In Young's double slit experiment radiations fromtwo coherent sources of intensity I and 4 Isuperimpose. The ratio of intensity at the maximumto minimum will be :

a) 5 : 3 b) 9 : 1

c) 4 : 1 d) 2 : 1

55. In Young's double slit experiment if the slit widthsare in the ratio of 1 : 9, the ratio of intensity at themaximum to that at the minimum will be :

a) 10 : 9 b) 9 : 1

c) 4 : 1 d) 3 : 1

56. In Young's experiment the wavelength of red lightis 7.5 10–5 cm and that of blue light is 5.0 10–5

cm. The value of n for which (n + 1)th blue brightband coincides with nth red band is :

a) 8 b) 4

c) 2 d) 1





57. In Young's double slit experiment the 7th maximumwith wavelength

1 is at a distance d

1 and that

with wavelength 2 is at distance d

2. Then 1

2

d

d

a) 1

2

b) 2

1

c)

21

22

d)

222

1

58. In a double slit experiment, the slits are separatedby a distance d and the screen is at a distance Dfrom the slits. If a maximum is formed just oppositeto each slit, then what is the order of the fringeso formed ?

a) 2d

2 Db)

22d

D

c) 2d

Dd)

2d

4 D

59. Two beams of light of intensities 4 I & I interfere.The intensity on the screen, where the phase

difference is ,2

will be :

a) 5 I b) 6 I

c) 8 I d) 9 I

60. In Young's double slit experiment if the slit widthsare in the ratio 1 : 9, the ratio of the intensity atminima to that at maxima will be :

a) 1 b) 1

9

c) 1

4d)

1

3

61. Two coherent light sources s1 & s

2 ( = 6000 ft.)

are 1 nun apart from each other. The screen isplaced at a distance of 25 cm from the sources.The width of the fringes on the screen shouldbe :

a) 0.015 cm b) 0.025 cm

c) 0.010 cm d) 0.030 cm

62. Light from two coherent sources of same amplitude(A) and wavelength () illuminates the screen.The intensity of central maximum is I

0. If the

source were incoherent, the intensity at the samepoint will be :

a) 0I

2b) I

0

c) 2 I0

d) 4 I0

63. Young's double slit experiment is performed withlight of wavelength 550 nm. The separationbetween the slits is 1.10 mm and the screen isplaced at a distance of 1 m. What is the distancebetween the consecutive bright and dark fringes ?

a) 0.5 mm b) 1.0 mm

c) 1.5 mm d) none of the above

64. Two light beams of intensities I1 and I

2 produce

an interference pattern. The contrast betweenthe fringes is the best when :

a) 21

II

4 b) 2

1

II

3

c) 21

II

2 d) 1 2I I

65. In an interference pattern produced by twoidentical slits, the intensity at the site of maximumis I. When either slit is closed, the intensity at thesame spot is I

0, What is the relation between I

and I0

?

a) I = I0

b) I = 2 I0

c) I = 4 I0

d) I = 16 I0

66. The rrummum thickness of air film which appearsdark under green light of wavelength 500 nm is :

a) 25 nm b) 50 nm

c) 250 nm d) 500 nm

67. Two slits separated by a distance of 1 mm areilluminated with light of wavelength 6.0 10–7 m.The interference fringes observed on a screenplaced 1m from the slits. The distance betweenthird dark fringe and the fifth bright fringe is equalto :

a) 0.60 mm b) 1.50 mm

c) 3.00 mm d) 4. 50 mm

68. The radius of nth half period zone is proportionalto :

a) n2 b) n

c) n d) 1 / n

69. What should be the number of half period zoneto obtain the first minimum ?

a) 4 b) 3

c) 2 d) 1





Diffraction of Light :

70. How will the diffraction pattern of single slitchange when yellow light is replaced by bluelight ? The fringes will be :

a) wider b) narrower

c) brighter d) fainter

71. What will be the angle of diffraction for the firstminimum due to Fraunhoffer diffraction withsources of light of wavelength 550 nm and slit ofwidth 0.55 mm ?

a) 1 rad b) 0.1 rad

c) 0.01 rad d) 0.001 rad

72. The fringe width of the diffraction pattern andthe slit width d are related as :

a) d b) 1

d

c) d d) is independent of d

73. Which of the following is one of the essentialconditions for observing Fresnel class of diffraction ?

a) Both source and screen at infinite distancefrom the slit

b) Either screen or source at inifinite distancefrom the slit

c) Neither screen nor source at infinite distancefrom the slit

d) None of the above

74. Prof. Raman was awarded nobel prize for hiswork connected with which of following pheno-menon of radiations ?

a) Scattering b) Diffraction

c) Interference d) Polarisation

75. Which of the following is essential for observingdiffraction ?

a) Two coherent source

b) A narrow slit

c) A screen d) White light

76. The interference differs from diffraction in that :

a) it cannot be observed with white light

b) unlike diffraction the interference fringes areof varying intensity

c) interference minima are perfectly dark andthat of diffraction may not be dark

d) the diffraction fringes are of equal width butthe interference fringes are of unequal width

77. The width of the diffraction fringe varies :

a) directly as the distance between the slit andthe screen

b) inversely as the wavelength

c) directly as the width of the slit

d) inversely as the size of the source from whichthe slit is illuminated

78. Diffraction pattern cannot be observed with :

a) two narrow slits

b) large number of narrow slits

c) one narrow slit

d) one wide slit

79. Which of the following cannot produce colourswith white light ?

a) Dispersion b) Interference

c) Diffraction d) Polarisation

80. What is the angular spread of the first maximumand the central maximum when diffraction isobserved with 540 nm light through a slit of width1 mm ?

a) 270 10–6 rad b) 540 10–6 rad

c) 810 10–6 rad d) 1080 10–6 rad

81. A plane wave front of wavelength is incidenton a single slit of width b. What is the angularwidth for secondary maximum ?

a) 2b

b)

b

c) 2

b

d)

b

82. What is the phase difference between the consecutivehalf period zones ?

a) 4

b)

2

c) d) 2

83. A narrow circular disc is placed in the path ofbeam of white light. The centre of the geometricalshadow will be :

a) dark b) bright

c) partially bright d) couloured

84. What happens to the width of central diffractionmaximum when the width of the aperture increases?

a) Increases b) Decreases

c) Unchanged d) Cannot be predicted





85. Which property of light is confirmed by diffraction?

a) Wave nature

b) Transverse wave nature

c) Longitudinal wave nature

d) Quantum nature

86. Which of the following undergoes largest diffraction?

a) y-rays b) Ultraviolet light

c) Infra red light d) Radio waves

87. Light of wave length 600 nm is incident on a singleslit. The first minimum of the diffraction patternis obtained at a distance of 4 mm from the centre.The distance between the screen and the slit is2 m. What is the width of the slit ?

a) 0.1 mm b) 0.3 m

c) 0.5 m d) 0.6 mm

88. Light of wavelength 500 nm is incident on a slitof width 0.1 mm. The width of the central brightline on the screen is 2 m. What is the distance ofthe screen ?

a) 0.5 m b) 0.75 m

c) 1 m d) 2 m

Recent Questions from MH-CET Exams :

89. In Young's experiment if d = 1 mm, D = 1 m,X = 6 mm then wavelength of light used is :

a) 6000 Å b) 4000 Å

c) 5000 Å d) 4850 Å

90. In an interference pattern the phase differenceof the waves reaching a dark point in radians, is :

a) 0 b) 2

c) d) 2

91. In a biprism experiment the band width is 0.4 mmwhen the eye piece is at a distance of 1 m fromthe slit. If the eye piece is moved through adistance of 25 cm towards the biprism, withoutchanging any other arrangement then the changein fringe width is :

a) 0.0001 mm b) 0.001 mm

c) 0.01 mm d) 0.1 mm

92. In Young's experiment the distance of the nth

bright band from the central bright band in termsof band width '' is :

a) 2 n b) n

c) (n + 1) d) (2n + 1)

93. In a biprism experiment with yellow light( = 590 nm) 62 fringes are obtained in a givenregion. The number of fringes obtained with greenlight ( = 550 nm) will be :

a) 52 b) 62

c) 67 d) 76

94. Which of the following graphs shows the variationof fringe width of interference pattern of doubleslit with frequency of light :

a) b)

c) d)

95. Two waves having intensity ratio 9 : 1 produceinterference. The ratio of maximum to minimumintensity is :

a) 10 : 8 b) 9 : 1

c) 4 : 1 d) 2 : 1

96. In Young's experiment for interference of lightwith two slits, reinforcement takes place when

nsin ,

d

here d is :

a) Distance from slits to screen

b) Distance between dark and bright fringes

c) Distance between slits

d) Width of nth fringe

97. A light of wavelength 550 nm in a biprismexperiment gives 67 fringes in a given region.When a light of wavelength 442 nm is used, thenumber of fringes in the same region will be :

a) 45 b) 54

c) 76 d) 83

98. In Young's double slit experiment carried out withlight of wavelength = 5000 Å the distance betweenthe slits is 0.2 mm and screen is 2.0 m away fromthe slits. The centre maxima is at n = 0. The thirdmaximum will be at a distance x (from centralmaxima) equal to :





a) 5 cm b) 0.5 cm

c) 1.67 cm d) 1.5 cm

99. In biprism experiment band width can be increasedby :

a) decreasing distance between virtual sources

b) increasing distance between virtual sources

c) increasing distance of real source

d) none of these

100. Condition for destructive interference is :

a) 0, 2, 4 b) , 3, 5

c) 0, , 22

d) none of these

101. In the biprism experiment green light of wave-length 414 Å is replaced by red light of wave-length 518 Å. It is found that nth bright point ofred light coincide with (n + 1)th bright point ofgreen light. Value of n is :

a) 6 b) 5

c) 4 d) 3

102. If I1 : I

2 = 100 : 1 what is the ratio of maximum

and minimum intensity ?

a) 11 : 9 b) 121 : 81

c) 11 : 3 d) 81 : 12l

103. In Young's double slit experiment, a minimum isobtained when the phase difference of superimposing waves is :

a) zero b) (2n – 1) c) n d) (n + 1)

104. For minima to take place between two mono-chromatic light waves of wavelength , the pathdifference should be :

a) n b) (2n 1) / 4

c) (2n 1) / 2 d) (2n 1)

105. With a monochromatic light the fringe widthobtained in a double slit experiment is 0.8 mm. Ifthe whole set up is dipped in water of refractiveindex 1.6, new fringe width is :

a) 0.6 mm b) 0.5 mm

c) 0.8 mm d) 0.9 mm

106. The optical path difference between two pointsis 72 m and light wavelength 6000 Å is arrivingat that point then the point is :

a) bright b) dark

c) neither bright nor dark

d) can't say

107. In a double slit experiment keeping other arrange-ment same, if wavelength increases from 5000 Åto 10,000 Å, the fringe width :

a) becomes twice the initial value

b) remains same

c) disappears d) becomes half

108. For two coherent sources, path diff. = 5 . If thedistance of this point is 0.5 mm, then the fringewidth is :

a) 0.05 mm b) 0.1 mm

c) 0.8 mm d) 1.2 mm

109. Wavelength of light is 5500 Å and distancebetween slit and screen is 1 m and distancebetween the sources is 1.1 m ; then distancebetween bright and dark fringe is :

a) 2.5 10–4 m b) 2.5 10–3 m

c) 0.25 m d) 2.50 m

110. If intensity of one of the coherent sources isreduced, then intensity of dark bands will :

a) reduce b) increase

c) remain the same d) none of these

111. Monochromatic light means, light having :

a) different wavelength

b) same wavelength

c) unidirectional

d) coherent wavelengths

112. If the distance of a point on the screen from thetwo sources is 1.8 10–5 m and 1.23 10–5 mand wavelength of light used is 6000 Å then, it is :

a) 9th dark fringe b) 10th dark fringe

c) 10th bright fringe d) 11th dark fringe

113. Two sources of light 0.5 mm apart are placed ata distance of 1.2 m and wavelength of light is5000 A. The phase difference between two lightwaves interfering on the screen at a point at adistance 3 mm from central bright band is :

a) 5 radian b) 2 radian

c) 3 radian d) 6 radian

114. In the interference experiment with a biprism, thedistance of the slit from the screen is increasedby 25% and the separation between the slits ishalved. If -represents the original bandwidth, thenew bandwidth is :

a) 2.5 b) 1.5

c) 2 d) 4





115. In interference of light, a point is bright if the pathdifference between the two beams arriving at thatpoint is :

a) an integral multiple of the wavelength

b) an odd multiple of the wavelength

c) an odd multiple of half of the wavelength

d) an even multiple of the wavelength

116. In the biprism experiment keeping the experimentalset up unchanged the fringe width :

a) increases with increase in wavelength

b) decreases with increase in wavelength

c) increases with decreases in wavelength

d) remains unchanged with change in wavelength

117. In the Young's double slit experiment the fringewidth is found to be 2 mm, when light of wave-length 6000 Å is used. Find the change in fringewidth if the whole apparatus is immersed in waterof refractive index 1.33 :

a) 0.5 mm b) 1 mm

c) 1.5 mm d) 2 mm

118. If the aperature of a telescope is decreased theresolving power will :

a) increase b) decrease

c) remain same d) zero

119. In an interference experiment, phase differencefor points where the intensity is minimum is(n = 1, 2, 3, ......)

a) n b) (n + 1)

c) (2n – 1) d) zero

120. If the ratio of amplitude of two waves is 4 : 3,then the ratio of maximum & minimum intensitiesis :

a) 16 : 9 b) 9 : 16

c) 1 : 49 d) 49 : 1

121. If a torch is used in place of monochromatic lightin Young's experiment, what will happen ?

a) no fringe will appear

b) only bright fringe will appear

c) fringe will occur as from monochromaticsource

d) fringe will appear for a moment and then itwill disappear

122. Fringes are produced with monochromatic lightof wavelength 5.45 10–5 cm. A thin glass plate

of R.I. 1.5 is then normally placed in one of thepaths of interfering waves and the central brightband of the fringe system is found to move intothe position, previously occupied by the third brightband from the system. The thickness of glass platewill be :

a) 32.7 10–4 cm b) 32.7 10–5 cm

c) 16.7 10–5 cm d) 12.5 10–5 cm

123. Light of wavelength 5000 Å is incident normallyon a slit. The first minimum of the diffractionpattern is formed at a distance of 5 mm fromcentral maximum. The screen is situated at adistance of 2 m from the slit. The slit width is :

a) 0.2 mm b) 0.8 mm

c) 0.4 mm d) 2.0 mm

124. Light of wavelength 6328 Å is incident normallyon a slit of width 0.2 mm. The angular width ofcentral maximum on a screen distant 9 m will be :

a) 0.36° b) 0.18°

c) 0.82° d) 0.09°

125. In two similar interference experiments, wave-lengths of light used were in the ratio 4 : 5, thesource to screen distances were in the ratio 5 : 6and the source separation in the ratio 2 : 3. Theratio of their band widths is :

a) 1 : 1 b) 1 : 2

c) 2 : 1 d) 1 : 4

126. For minimum intensity the phase differencebetween the two waves is :

a) 2 n b) (2n – 1)

c) (2n + 3) d) n

127. Two waves of amplitudes A1 & A

2 superimpose

with each other such that A1 > A

2. The difference

between maximum and minimum amplitudes is :

a) A1

b) 2 A2

c) 2 A1

d) A2

128. In a single slit diffraction pattern intensity andwidth of fringes are :

a) unequal width

b) equal width

c) equal width and equal intensity

d) unequal width and unequal intensity

129. If path difference between waves is 11

4

then

the phase difference between two waves will be :





a) 11

2

b)

5

2

c) 13

2

d)

7

2

130. Resolving power of telescope can be increasedby :

a) increasing diameter of the objective of thetelescope

b) decreasing diameter of the objective of thetelescope

c) increasing by wavelength of light

d) none of these

131. In the experiment of interference, p is the numberof bright bands for a light of wavelength

1. If

source of light is replaced by 2 then the number

of bright bands will be :

a) 2

1

p

b) 1

2

p

c) 1p d) 2p

132. In Young's double slit experiment, the slits are2 mm apart and are illuminated by photons oftwowavelength

1 = 12000 Å and

2 = 1000 Å. At

what minimum distance from the common centralbright fringe on the screen 2 m from the slit will abright fringe from one interference patterncoincide with a bright fringe from the other ?

a) 6 mm

b) 4 mm

c) 3 mm

d) 8 mm

133. A parallel beam of fast moving electrons is incidentnormally on a narrow slit. A fluorescent screen isplaced at a large distance from the slit. If thespeed of the electrons is increased, which of thefollowing statements is correct ?

a) the angular width of the central maximum ofthe diffraction pattern will increase

b) the angular width of the central maximum ofthe diffraction pattern will decrease

c) the angular width of the central maximum willdecrease

d) diffraction pattern is not observed on the screenin the case of electrons

134. Two coherent sources of intensity ratio '' interfere.

In interference pattern max min

max min

I I

I I

a) 2

1

b) 2

1

c) 2

1

d)

1

2

135. A light is travelling from air into a medium.Velocity of light in a medium is reduced to 0.75times the velocity in air. Assume that angle ofincidence 'i' is very small, the deviation of the rayis :

a) i b) i

3

c) i

4d)

3i

4

136. For the same angle of incidence, the angles ofrefraction in media 'P', 'Q', 'R' and 'S' are 50°,40°, 30°, 20° respectively. The speed of light isminimum in medium :

a) P b) Q

c) R d) S





REVISION OUESTIONS

from Competitive Exams

1. If yellow light emitted by sodium lamp in Young'sdouble slit experiment is replaced by mono-chromatic blue light of the same intensity :

a) fringe width will decrease

b) fringe width will increase

c) the fringe width will remain unchanged

d) fringes will become less intense

2. In a Young's experiment, two coherent sourcesare placed 0.90 mm apart and the fringes areobserved one metre away. If it produces thesecond dark fringe at a distance of 1 mm fromthe central fringe, the wavelength of mono-chromatic light used would be :

a) 60 10–4 cm b) 10 10–4 cm

c) 10 10–5 cm d) 6 10–5 cm

3. In Young's double slit experiment, carried out withlight of wavelength = 5000 Å, the distancebetween the slits is 0.2 mm and the screen is at200 cm from the slits. The central maximum is atx = 0. The third maximum (taking the centralmaximum as zeroth maximum) will be at x equalto :

a) 1.67 cm b) 1.5 cm

c) 0.5 cm d) 5.0 cm

4. Interference was observed in interference chamber,when air was present, now the chamber isevacuated, and if the same light is used, a carefulobserver will see :

a) no interference

b) interference with bright bands

c) interference with dark bands

d) interference in which breadth of the fringe willbe slightly increased

5. Ratio of intensities of two waves are given by4 : 1. Then ratio of the amplitudes of the twowaves is :

a) 2 : 1 b) 1 : 2

c) 4 : 1 d) 1 : 4

6. The frequency of e.m. wave which best suitedto observe a particle of radii 3 10–4 cm is of theorder of :

a) 1015 b) 1013

c) 1014 d) 1012

7. Interference pattern is obtained on a screen dueto two identical coherent sources of mono-chromatic light. The intensity of the central brightfringe is 1. When one of the sources is blocked,its intensity becomes I

0, The intensity in two

situations is related as :

a) I = I0

b) I = 2 I0

c) I = 3 I0

d) I = 4 I0

8. In Young's double slit experiment the distancebetween the two slits is 0.1 mm and the wavelengthof light used is 4 10–7 m. If the width of thefringe on the screen is 4 mm, the distance betweenscreen and slit is :

a) 0.1 mm b) 1 cm

c) 0.1 cm d) 1 m

e) 10 m

9. The light from a sodium vapour lamp passesthrough a single narrow slit and then through twoclose, parallel narrow slits. The single slit lies onthe perpendicular bisector of the line joining thetwo close, parallel slits. An observer lookingtowards the lamp through the double slit will see :

a) A continuous yellow band

b) A continuous black band

c) Alternate black and yellow bands

d) A coloured spectrum

10. In the Young's double slit experiment, the intensityon the screen at a point where path difference is is K. What will be the intensity at the pointwhere path difference is /4 ?

a) K

4b)

K

2

c) K d) zero

11. In the Young's double slit experiment using sodiumlight (/5898 Å), 92 fringes are seen. If greencolour (= 5461) is used, how many fringes willbe seen ?

a) 62 b) 67

c) 85 d) 99

12. In the Young's double slit experiment a mica slipof thickness t and refractive index is introducedin the ray from the first source S

1. By how much

distance the fringes pattern will be displaced ?

a) d

( 1) tD

b) D

( 1) td





c) d

( 1) D d) D

( 1)d

13. The intensity of the central fringe due to the inter-ference of light from two similar slits is I. Whenone of the slit is closed, the intensity is I

0. What is

the relation between I and I0 ?

a) 0I 4 I b) 0I 2 I

d) 0I I d) 0II

2

14. The displacement of the interfering light waves

are 1y 4sin t and 2y 3sin t .2

What

is the amplitude of the resultant wave ?

a) 5 b) 7

c) 1 d) zero

15. For the sustained interference of light, the necessarycondition is that the two sources should :

a) have constant phase difference

b) be narrow

c) be close to each other

d) of same amplitude

16. In two separate setups of the Young's double slitexperiment, fringes of equal width are observedwhen lights of wavelengths in the ratio 1 : 2 areused. If the ratio of the slit separation in the twocases is 2 : 1, the ratio of the distances, betweenthe plane of the slits and the screen, in the twosetups is :

a) 4 : 1 b) 1 : 1

c) 1 : 4 d) 2 : 1

17. Light waves projected on oil surface shows 7colours due to :

a) Polarisation b) Diffraction

c) Refraction d) Interference

18. In Young's double slit experiment distance betweensources is 1 mm and distance between the screenand source is 1 m. If the fringe width on the screenis 0.06 cm then =

a) 6000 Å b) 4000 Å

c) 1200 Å d) 2400 Å

19. If the distance between a point source and screenis made one third, then intensity of light (I) andthe initial intensity of light will be related as :

a) 0II

3 b) 0I

I9

c) 0I I d)

0I 9 I

20. The equation of displacement of two waves aregiven as,

1y 10sin 3 t3

2y 5 sin3 t 3 cos3 t

then what is the ratio of their amplitudes ?

a) 1 : 2 b) 2 : 1

c) 1 : 1 d) none of these

21. Two waves of intensities I & 4 I superpose, thenthe maximum and minimum intensities are :

a) 5 I and 3 I b) 9 I and I

c) 9 I and 3 I d) 5 I and I

22. Intensity of two waves, which produces diffractionare 9 : 4. The ratio of maximum and minimumintensity is :

a) 9 : 4 b) 3 : 2

c) 25 : 1 d) 5 : 1

23. If light of low wavelength is used in Young'sdouble slit experiment then width of the fringewill :

a) decrease b) increase

c) not fixed d) no change

24. Two sources of light are said to be coherent whenthe waves produced by them have :

a) equal wavelength

b) equal amplitude

c) equal wavelength & constant phase difference

d) amplitude and wavelength are same

25. If the ratio of amplitude of two waves is 4 : 3,then the ratio of maximum and minimum intensityis :

a) 16 : 18 b) 18 : 16

c) 49 : 1 d) 94 : 1

26. Two plane mirrors inclined each other with 600.The number of images formed will be :

a) 6 b) 12

c) 5 d) 10





27. In a biprism experiment, by using light of wave-length 5000 Å, 5 mm wide fringes are obtainedon a screen 1.0 m away from the coherent sources.The separation between the two coherent sourcesis :

a) 1.0 mm b) 0.1 mm

c) 0.05 mm d) 0.01 mm

28. In the Young's double slit experiment, if the phasedifference between the two waves interfering ata point is , the intensity at that point can beexpressed by the expression :

a) 2 2 2I A B cos

b)A

I cosB

c) I A Bcos2

d) I A B cos

[where A and B depend upon the amplitudes ofthe two waves]

29. In interference pattern, by two identical slits, theintensity of central maximum is I. What will bethe intensity at the same spot, if one of the slits isclosed ?

a) I

4b) I

c) I

2d) 2 I

30. In an experiment to demonstrate the interferenceof light using Young's slits, the separation of twonarrow slits is doubled in order to maintain thesame spacing of the fringes. The distance 'd' ofthe screen from the slits must now be altered toabout :

a) 2 d b) d

c) d

2d)

d

2

31. In the Young's double slit experiment the separationbetween the slits is halved and the distancebetween the slits and the screen is doubled. Thefringe width is :

a) halved b) unchanged


32. In a certain double slit experiment arrangement,interference fringes of width 1.0 mm each areobserved when light of wavelength 5000 Å. isused keeping the set up unaltered if the source isreplaced by another wavelength 6000 Å, thefringe width will be :

a) 0.5 mm b) 1.0 mm

c) 1.2 mm d) 1.5 mm

33. In Young's double slit experiment, the separationbetween the slit is halved and distance betweenthe slit and screen is doubled. The fringe widthis :

a) unchanged b) halved


34. Two waves having intensity in the ratio 25 : 4 produceinterference. The ratio of the maximum to theminimum intensity is :

a) 5 : 2 b) 7 : 3

c) 49 : 9 d) 9 : 49

35. Two waves,

1 1 1y A sin( t ) and 2 2 2y A sin( t )

superimpose to form a resultant wave whoseamplitude is :

a) 2 21 2 1 2 1 2A A 2A A cos( )

b) 2 21 2 1 2 1 2A A 2A A sin( )

c) 1 2A A d) 1 2A A

36. In Young's experiment, fringe width was foundto be 0.4 mm. If whole apparatus is immersed in

water of refractive index 4

( ) ,3

new fringe

width is :

a) 0.25 mm b) 0.33 mm

c) 0.40 mm d) 0.53 mm

37. Fraunhoffer lines become brighter during solareclipse, because :

a) the sun-rays get diffracted by moon

b) solar radiations are completely blocked off bythe moon and only the photosphere radiationsreach the earth

c) chromosphere radiations are stopped by themoon and only the photosphere radiationsreach the earth





d) photosphere radiations are stopped by themoon, but the chromosphere radiations areable to reach the earth

38. The resolving power of a telescope can be increasedby :

a) increasing diameter of objective lens

b) decreasing diameter of objective lens

c) increasing diameter of eyepiece

d) decreasing diameter of eyepiece

39. In diffraction grating experiment, the pattern canbe enhanced by :

a) increasing wavelength

b) decreasing wavelength

c) constant wavelength


40. A parallel beam of monochromatic light is incidentnormally on a slit. The diffraction pattern isobserved on a screen placed at the focal plane ofa convex lens. If the slit width is increased, thecentral maximum of the diffraction pattern will :

a) become broader and fainter

b) become broader and brighter

c) become narrower and fainter

d) become narrower and brighter

e) remain unchanged

41. If the red light is replaced by blue light illuminatingthe object in a microscope the resolving powerof the microscope :

a) decreases b) increases

c) gets halved d) remains unchanged

e) becomes 1/4 of the original value

42 A limit on the performance of a resolvinginstrument is set by :

a) Quantum nature of light

b) Interference of light

c) Diffraction of light

d) Polarisation of light

43. Angular width () of central maximum of adiffraction pattern of a single slit does not dependupon :

a) distance between slit and source

b) wavelength of light used

c) width of the slit

d) frequency of light used

44. Wavelengths of light used in an optical instrumentare

1 = 4000 Å and

2 = 5000 Å, then ratio of

their respective resolving powers (correspondingto

1 and

2) is :

a) 16 : 25 b) 9 : 1

c) 4 : 5 d) 5 : 4

45. Two points separated by a distance of 0.1 mmcan just be resolved in a microscope when a lightof wavelength 6000 Å is used. If the light ofwavelength 4800 Å is used, this limit of resolutionbecomes :

a) 0.08 mm b) 0.10 mm

c) 0.12 mm d) 0.06 mm

46. To observe diffraction, the size of an obstacle :

a) should be of the same order as wavelength

b) should be much larger than the wavelength

c) has no relation with wavelength

d) should be exactly /2

47. The resolving power of a telescope whose lenshas a diameter of 1.22 m for a wavelength of5000 Å is :

a) 2 105 b) 2 106

c) 2 d) 2 102

e) 2 104

48. Yellow light is used in single slit diffractionexperiment with slit width 0.6 mm. If yellow lightis replaced by X-rays, then the pattern will reveal :

a) less number of fringes

b) more number of fringes

c) no diffraction pattern

d) that the central maxima is narrower

49. In diffraction from a single slit, the angular widthof the central maxima does not depend on :

a) wavelength of light used

b) width of slit

c) distance of slits from screen

d) ratio of Iv and slit width

50. The resolving power of a reflecting telescopedepends :

a) on the intensity of light used

b) directly on wavelength of the light used

c) on the focal length of objective lens

d) directly on the diameter of objective lens





51. When a compact disc is illuminated by a sourceof white light, coloured lines are observed. Thisis due to :

a) dispersion b) diffraction

c) interference d) refraction

52. When light is incident on a diffraction grating thezero order principal maximum will be :

a) spectrum of the colours

b) white

c) one of the component colours

d) absent

53. A beam of light of wavelength 600 nm from asource falls on a single slit 1 mm wide and theresulting diffraction pattern is observed on ascreen 2 m away. The distance between the firstdark fringes on either side of the central brightfringe is :

a) 2.4 cm b) 2.4 mm

c) 1.2 mm d) 1.2 cm

54. In Young's double slit experiment, one of the slitis wider than other, so that the amplitude of thelight from one slit is double of that from other slit.If 1 m be maximum intensity, the resultantintensity I when they interfere at phase difference is given by :

a) 2mI

1 8cos9 2

b) mI4 5cos

9 2

c) 2mI

1 2cos3 2

d) 2mI1 4cos

5 2

55. Two coherent point sources S1 & S

2 are separated

by a small distance 'd' as shown. The fringesobtained on the screen will be :

a) concentric circle b) points

c) straight lines d) semi-circles

56. In the Young's double slit experiment using amonochromatic light of wavelength , the pathdifference (in terms of an integer n) correspondingto any point having half the peak intensity is :

a) (2n 1)2

b) (2n 1)

4

c) (2n 1)8

d) (2n 1)

16

57. A parallel beam of fast moving electrons is incidentnormally on a narrow slit. A fluorescent screen isplaced at a large distance from the. slit. If thespeed of the electrons is increased, which of thefollowing statements is correct ?

a) The angular width of central maximum willdecrease.

b) The angular width of central maximum willbe unaffected.

c) Diffraction pattern is not observed on thescreen in case of electrons.

d) The angular width of central maximum willincrease.

58. In Young's double slit experiment, the slits are2 mm apart and are illuminated by photons of twowavelengths

1 = 12000 Ao and

2 = 10000 Ao.

At what minimum distance from the commoncentral bright fringe on the screen 2 m from theslit will a bright fringe from one interferencepattern coincide with a bright fringe from theother ?

a) 3 mm b) 8 mm

c) 4 mm d) 6 mm

59. In the Young's double-slit experiment, the intensityof light at a point on the screen where the pathdifference is is K, (being the wave length oflight used). The intensity at a point where the

path difference is ,4

will be :

a) zero b) K

c) K

4d)

K

2

60. A beam of light = 600 nm from a distant sourcefalls on a single slit 1 mm wide and the resultingdiffraction pattern is observed on a screen 2 maway. The distance between first dark fringeson either side of the central bright fringe is :

a) 2.4 mm b) 1.2 cm

c) 1.2 mm d) 2.4 cm





61. A light source, which emits two wavelengths

1= 400 nm and

2 = 600 nm, is used in a Young's

double slit experiment. If recorded fringe widthfor

1 and

2 are

1 and

2 and the number of

fringes for them within a distance y on one sideof the central maximum are m

1 & m

2, respectively,

then :

a) 2 1

b) 1 2m m

c) From the central maximum, 3rd maximum of

2 overlaps with 5th minimum of

1

d) The angular separation of fringes for 1 is

greater than 2

62. In a double slit experiment, the two slits are 1 mmapart and the screen is placed 1 m away. Amonochromatic light of wavelength 500 nm isused. What will be the width of each slit forobtaining ten maxima of double slit within thecentral maxima of single slit pattern ?

a) 0.5 mm b) 0.02 mm

c) 0.2 mm d) 0.1 mm

63. For a parallel beam of monochromatic light ofwavelength '', diffraction is produced by a singleslit whose width 'a' is of the order of the wave-length of the light. If 'D' is the distance of thescreen from the slit, the width of the centralmaxima will be :

a) Da

b)

2Da

c) 2D

a

d)

D

a

64. On a hot summer night, the refractive index ofair is smallest near the ground and increases withheight from the ground. When a light beam isdirected horizontally, the Huygen's principle leadsus to conclude that as it travels, the light beam :

a) bends downwards

b) bends upwards

c) becomes narrower

d) goes horizontally without any deflection

65. Two slits in Youngs experiment have widths inthe ratio 1 : 25. The ratio of intensity at the maxima

and minima in the interference pattern, max

min

I

I is :

a) 49

121b)

4

9

c) 9

4d)

121

49

66. At the first minima adjacent to the central maximumof a single-slit diffraction pattern, the phasedifference between the Huygen's wavelet fromthe edge of the slit and the wavelet from themidpoint of the slit is :

a) radian b) 8

radian

c) 4

radian d)

2

radian





Brain Teasers

1. A Young's double slit experiment is conductedwith light of wavelength . The intensity of thebright fringe is I

0. The intensity at a point where

path difference is /4 is given by :

a) zero b) 0I

4

c) 0I

6d)

0I

2

2. The intensity ratio of the two interfering beams

of light is . What is the value of max min

max min

I I?

I I

a) 2 b) 2

1

c) 2

1 d) 1

2

3. In Young's double slit experiment the 8th maximumwith wavelength

1 is at a distance d

1 from the

central maximum. And the 6th maximum with

wavelength 2 is at a distance d

2. Then 1

2

d

d

a) 2

1

4

3

b) 1

2

4

3

c) 2

1

3

4

d) 1

2

3

4

4. In Young's double slit interference experiment,

the distance between two sources is 0.1

mm. The

distance of the screen from the sources is 25 cm.Wavelength of light used is 5000 Å. Then theangular position of the first dark fringe is :

a) 0.10° b) 0.15°

c) 0.30° d) 0.45°

5. When the distance between the object and thescreen is more than 4f, we can obtain image ofthe object on the screen for the two positions ofthe lens. It is called displacement method. In onecase the image is magnified and in the other caseit is diminished. If I

1 and I

2 be the sizes of the

two images, then the size of the object is :

a) 1 2I I

2

b) 1 2I I

c) 1 2I I d) 1

2

I

I

6. A light source is located at P1 as shown in the

figure. All sides of the polygon are equal. Theintensity of illumination at P

2 is I

0, What will be

the intensity of illumination at P3

?

a) 03 3 I

8b)

0I

8

c) 0

3I

8d) 0

3I

8

7. The ratio of amplitude of coherent sources is 2 : 1in a Young's double slit experiment. What is theratio of maximum and minimum intensities of thefringes ?

a) 1 : 1 b) 2 : 1

c) 4 : 1 d) 9 : 1

8. By what distance seventh maxima changes fromd

1 and d

2 if wavelength of light changes from

1

to 2 in Young's double slit experiment ?

a) 2

1 12

2 2

d

d

b) 1 2

2 1

d

d

c) 1 1

2 2

d

d

d)

2

1 2

2 1

d

d

9. What is the ratio of maximum and minimumintensities if ratio of amplitude of two waves is4 : 3 ?

a) 49 : 1 b) 16 : 9

c) 9 : 16 d) none of these

10. If two waves have intensities in the ratio of 1 : 9.What is the ratio of maximum to minimum intensity?

a) 3 : 1 b) 4 : 1

c) 9 : 1 d) 1 : 4





11. If two wave having intensities I and 4 I aresuperimpossed then what are the maximum andminimum intensities ?

a) I and 9 I b) I and 4 I

c) 4 I and I d) 9 I and I

12. In Young's double slit experiment, what is theintensity at a point equidistant from the slit interms of maximum intensity I

0, if there is constant

phase difference between two sources as ?2

a) 04 I b) 03 I

4

c) 0I

2d) I

0

13. What is resultant intensity after interference oftwo coherent waves given as,

1 1y a cos wt and 2 2y a cos wt ?2

a) 1 2a a b) 2 21 2(a a )

c) 2a a d) 2 21 2(a a )

14. In two set ups of Young's double slit experiment,lights used have ratio of wavelength as 1 : 2 thenthe fringes are of equal width. If the ratio of thewidth of slits is 2 : 1 then what is the ratio ofdistances between slit and screen ?

a) 1 : 2 b) 4 : 1

c) 2 : 1 d) 1 : 4

15. Two waves intefere with each other what is theratio of their intensities (I

1: I

2) if ratio of resultant

maximum and minimum is 4 : 1 ?

a) 1 : 4 b) 9 : 1

c) 16 : 1 d) 4 : 1



DINESH [MHT - CET] PHYSICS


01. (d)

02. (b)

05. (c)

06. (c)

09. (a)

10. (b)

Answer Key

MH Text Book Based MCQ's

01. (a)

02. (d)

03. (c)

04. (b)

05. (c)

06. (d)

07. (b)

08. (b)

09. (c)

10. (b)

11. (c)

12. (c)

13. (a)

14. (b)

15. (b)

16. (d)

17. (d)

18. (c)

19. (c)

20. (c)

21. (d)

22. (a)

23. (c)

24. (b)

25. (a)

26. (b)

27. (d)

28. (c)

29. (d)

30. (a)

31. (c)

32. (d)

33. (b)

34. (a)

35. (b)

36. (a)

37. (a)

38. (b)

39. (a)

40. (b)

41. (a)

42. (a)

43. (a)

44. (c)

45. (a)

46. (c)

47. (c)

48. (c)

49. (a)

50. (d)

51. (d)

52. (b)

53. (c)

54. (b)

55. (c)

56. (c)

57. (a)

58. (a)

59. (a)

60. (c)

61. (a)

62. (a)

63. (d)

64. (d)

65. (c)

66. (c)

67. (b)

68. (c)

69. (c)

70. (b)

71. (d)

72. (b)

73. (c)

74. (a)

75. (b)

76. (c)

77. (a)

78. (d)

79. (d)

80. (c)

81. (b)

82. (c)

83. (b)

84. (b)

85. (a)

86. (d)

87. (b)

88. (d)

89. (a)

90. (c)

91. (d)

92. (b)

93. (c)

94. (b)

95. (c)

96. (c)

97. (d)

98. (d)

99. (a)

100. (b)

101. (c)

102. (b)

103. (b)

104. (c)

105. (b)

106. (a)

107. (a)

108. (b)

109. (a)

110. (b)

111. (b)

112. (b)

113. (a)

114. (a)

115. (a)

116. (a)

117. (c)

118. (b)

119. (c)

120. (d)

121. (a)

122. (b)

123. (a)

124. (a)

125. (a)

126. (b)

127. (b)

128. (d)

129. (a)

130. (a)

131. (b)

132. (a)

133. (b)

134. (b)

135. (c)

136. (d)

REVISION QUESTIONS from Competitive Exams.

01. (a)

02. (d)

03. (b)

04. (d)

05. (a)

06. (a)

07. (d)

08. (d)

09. (c)

10. (b)

11. (d)

12. (b)

13. (a)

14. (a)

15. (a)

16. (a)

17. (d)

18. (a)

19. (d)

20. (c)

21. (b)

22. (c)

23. (a)

24. (c)

25. (c)

26. (c)

27. (b)

28. (d)

29. (a)

30. (a)

31. (d)

32. (c)

33. (b)

34. (c)

35. (a)

36. (d)

37. (b)

38. (a)

39. (a)

40. (d)

41. (b)

42. (c)

43. (a)

44. (d)

45. (a)

46. (a)

47. (b)

48. (d)

49. (c)

50. (d)

51. (b)

52. (b)

53. (b)

54. (a)

55. (a)

56. (b)

57. (a)

58. (d)

59. (d)

60. (a)

61. (a)

62. (a,b,c)

63. (c)

64. (b)

65. (c)

66. (a)

BRAIN TEASERS

03. (b)

04. (d)

07. (d)

08. (c)

11. (d)

12. (c)

13. (d)

14. (b)

15. (b)






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Interference & Diffraction