Light Reflection and Refraction

Submitted by :J Siddharth Jaya Sajeevan

Class: X-A

What makes things visible?

Object reflects light that falls on itReflected light received by our eyes

REFLECTION OF LIGHT

Laws of reflection of light:• The angle of incidence is equal to the angle of

reflection, and• The incident ray, the normal to the mirror at

the point of incidence and the reflected ray, all lie in the same plane

Plane Mirror

Properties of the image:• Virtual and erect• Size of the image = Size of the object• Image formed is as far behind the mirror as the object is in front of it• Laterally inverted

Spherical Mirrors

•Reflecting surface - curved inwards(Concave)/outwards(Convex)

•Pole(P)-Centre of the reflecting surface•Centre of curvature(C)-Centre of the sphere formed by the reflecting surface•Radius of curvature(R)-Radius of the sphere of which spherical mirror forms a part•Principal axis –Straight line passing through the pole & centre of curvature•Aperture-Diameter of the reflecting surface

Principal axis is normal to the mirror at its pole

Principal focus(F)-Point on principal axis where a beam of light parallel to principal axis converges to(Concave) or appears to diverge from after reflection from the spherical mirror(Convex).Focal length (f)-Distance between thepole & principal focus

Characteristics of Focus of a Concave and a Convex Mirror

Convex Mirror Concave Mirror The focus lies behind the mirror

The focus is in front of the mirror

The focus is virtual (rays of light after reflection appear to come from the focus)

The focus is real (rays of light after reflection converge at the focus)

R = 2f

• A ray parallel to the principal axis, after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror.

Image Formation by Spherical Mirrors

A ray passing through the principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis.

• A ray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror, after reflection, is reflected back along the same path.

Image Formation by Spherical Mirrors

A ray incident obliquely to the principal axis, towards a point P (pole of the mirror), on the concave mirror or a convex mirror, is reflected obliquely. The incident and reflected rays follow the laws of reflection at the point of incidence (point P), making equal angles with the principal axis.

Image formation by Concave Mirror

Image formation by Concave MirrorPosition of the object

Position of the image Size of the image Nature of the image

At infinity At the focus F Highly diminished, point-sized Real and inverted

Beyond C Between F and C Diminished Real and invertedAt C At C Same size Real and invertedBetween C and F Beyond C Enlarged Real and invertedAt F At infinity Highly enlarged Real and invertedBetween P and F Behind the mirror Enlarged Virtual and erect

Uses•Torches, search-lights and Vehicles headlights (powerful parallel beams of light)•Shaving mirrors (larger image of the face)•Enlarged images of the teeth of patients for dentists•Solar furnaces -Concentrate sunlight to produce heat

Image formation by Convex Mirror

Position of the object

Position of the image Size of the image Nature of the image

At infinity At the focus F, behind the mirror

Highly diminished, point-sized Virtual and erect

Between infinity and the pole P of the mirror

Between P and F, behind the mirror Diminished Virtual and erect

Uses:Rear-view (wing) mirrors•wider field of view •enable the driver to view much larger area than would be possible with a plane mirror.

Sign Convention for Reflection• The object is always placed to the left of the mirror. This implies that the light from the object

falls on the mirror from the left-hand side.• All distances parallel to the principal axis are measured from the pole of the mirror.• All the distances measured to the right of the origin (along + x-axis) are taken as positive while

those measured to the left of the origin (along – x-axis) are taken as negative.• Distances measured perpendicular to and above the principal axis (along + y-axis) are taken as

positive.• Distances measured perpendicular to and below the principal axis (along –y-axis) are taken as

negative.

Mirror Formula

MagnificationRelative extent to which the image of an object is magnified with respect to the object size

The magnification m can be expressed as in terms of object distance (u) andimage distance (v) :

REFRACTION OF LIGHTLaws of refraction of light:• The incident ray, the refracted ray and the normal to the interface of two transparent

media at the point of incidence, all lie in the same plane.• The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for

the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

If i is the angle of incidence and r is the angle of refraction, then, sin i / sin r = constant This constant value is called the refractive index of the second medium with respect to

the first

Refraction by Spherical Lenses

Image Formation by Lenses

Nature, position and relative size of the image formed by a concave lens for various positions of the object

Position of the object

Position of the image

Relative size of the image Nature of the image

At infinity At focus F1Highly diminished,

point-sized Virtual and erect

Between infinity and optical centre O of the

lens

Between focus F1 and optical centre O Diminished Virtual and erect

Nature, position and relative size of the image formed by a convex lens for various positions of the object

Position of the object Position of the image Relative size of the image Nature of the image

At infinity At focus F2Highly diminished, point-

sized Real and inverted

Beyond 2F1 Between F2 and 2F2 Diminished Real and invertedAt 2F1 At 2F2 Same size Real and inverted

Between F1 and 2F1 Beyond 2F2 Enlarged Real and inverted

At focus F1 At infinity Infinitely large or highly enlarged Real and inverted

Between focus F1and optical centre O

On the same side of the lens as the object Enlarged Virtual and erect

Sign Convention for Refraction• The object is always placed to the left of the mirror. This implies that the light from the object

falls on the mirror from the left-hand side.• All distances parallel to the optical axis are measured from the pole of the mirror.• All the distances measured to the right of the origin (along + x-axis) are taken as positive while

those measured to the left of the origin (along – x-axis) are taken as negative.• Distances measured perpendicular to and above the optical axis (along + y-axis) are taken as

positive.• Distances measured perpendicular to and below the optical axis (along –y-axis) are taken as

negative.

Lens Formula

• Ratio of the height of the image and the height of the object

Magnification

Magnification (m ) = h /h = v/u′

Power of a Lens• The degree of convergence or divergence of light rays

achieved by a lens is defined as Power of a lens.• Power of a lens is defined as the reciprocal of its focal length.• Represented by the letter P

P =1/f• SI unit–Dioptre(D)• 1 dioptre is the power of a lens whose focal length is 1 metre• Power of Convex lens-positive Concave lens-negative.