14 Total Internal Reflection

8/7/2019 14 Total Internal Reflection

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TOTAL INTERNALREFLECTIONTopic 14: Light

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TOTAL INTERNAL REFLECTION

A Review

Allthelighttravelingthroughanopticallydensermediumisreflectedinternallyatthepointofincidence.

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Critical Angle, c

Theangleofincidenceforwhichtheangleofrefractionintheopticallylessde

nsemediumis90degrees.

r = 90o

i = c

air

optically dense

medium

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Glass has a critical angle of 42 .

What does this sentence means?

o



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Relationship between Critical Angle and Refractive Index

r = 90o

i = c

n = sin i

sin rSo then,

n =

sin c

sin 90o

right ?

air

optically dense

medium



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Relationship between Critical Angle and Refractive Index

n 1

sin c

=



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Glass has a critical angle of 42 .o

Calculate the refractive index ofthis glass. Pr

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in Diamonds

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in SLR Cameras

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in Binoculars

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in Periscope

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in Rear Reflectors of Cars & Cycles

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TOTAL INTERNAL REFLECTIONin Optical Fibres

Thin, flexible, transparent fibers that act as a

waveguide, or "light pipe".

Transmitlightbetweenthetwoendsofthefibre

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A central glass core of high refractive index coated

with a thin layer of low refractive index glass.

Basic

struct

ure

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A light ray entering the optical fibre is totally

internally reflected if its angle of incidence is greater

than the critical angle of the optical fibre.

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N

N N

i > ci > c i > c

C C

C


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TOTAL INTERNAL REFLECTIONCurrent uses of Optical Fibres

In the bronchoscope, endoscopes, laproscopes.

Used by doctors and surgeons to view internal organs

through body orifices and small key hole incisions.

Medical ImagingPreparedbyCg



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Inspecting mechanical welds in pipes and engines

(airplanes, rockets, space shuttles, cars etc).

Mechanical ImagingPreparedbyCg



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Inspecting water and sewage pipes for blockages and

fractures

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TOTAL INTERNAL REFLECTIONCurrent Uses of Optical Fibres

Transmission of telephone calls & video calls,

tv broadcasts, internet and data transfer.

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TOTAL INTERNAL REFLECTIONAdvantages of using Optical Fibres over Copper cables.

Less

expensive

Less signal

degradation

Higher data

carrying capacity

Uses less power to

transmit signals

Thinner

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TOTAL INTERNAL REFLECTIONAdvantages of using Optical Fibres over Copper cables.

No electrical

interferenceNon-flamable

Ideal for

transmittingdigital signalsLightweight

Flexible

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TOTAL INTERNAL REFLECTIONConstructing the light path inside the fibre optics

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Given an incident ray entering the optical fibre,

draw the normal line at the point of incidence.

N


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Light ray is moving

from less optically dense to optically denser medium,

the light ray will bend towards the normal.

N


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If the refractive index of the fibre optic is given,

use a protractor to measure the angle of incidence,

use Snells law to calculate the angle of refraction and

then carefully draw the refracted light ray.

N


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Inside, the refracted ray now hits the boundary

between the two layers of glass.


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N

Draw the normal line again at the point of incidence.

It should be perpendicular to the surface boundary.


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N

i > c

Use a protractor to measure the angle of incidence.

If the angle of incidence > critical angle

Total Internal Reflection occurs instead of refraction.


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N

i

In reflection, angle of incidence = angle of reflection

Use a protractor to carefully measure out the angle of

reflection and construct the reflected light ray.

r


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N

N N

i > c

i > c i > cC CC

Repeat the steps until the light reaches the end of the

optical fibre.


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Draw the normal line at the point of incidence.

N


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Light ray is moving

from optically dense to less optically dense medium.

Light ray will bend away from the normal.

N


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N

N N

Your ray diagram may look something like this!

N

N

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14 Total Internal Reflection