Total internal reflection

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Total internal reflection:

Total internal reflection

PowerPoint Presentation:

The picture shows the image of a boy formed by a glass. the glass surface is not smooth enough. only very small amount of light is reflected by the glass. the environment outside is not dark enough. light ray should undergo refraction instead of reflection. dim image 

Introduction:

Introduction The inside surface of water or a glass block can act like a mirror.

1 Internal reflection:

1 Internal reflection A light ray hits the inside face of a semicircular block as follows. What will happen? air glass

1 Internal reflection:

1 Internal reflection The incident ray splits into 2 rays. angle of refraction < 90 o incident ray reflected ray refracted ray air glass For a small angle of incidence

1 Internal reflection:

1 Internal reflection incident ray reflected ray air glass C C as angle of incidence , angle of refraction  eventually, angle of refraction = 90  refracted ray angle of incidence = critical angle C

1 Internal reflection:

1 Internal reflection no refracted ray reflected ray air glass angle of incidence continues to , incident ray

1 Internal reflection:

1 Internal reflection Therefore, if angle of incidence > C , the light ray is totally reflected inside. reflected ray air glass incident ray This is called total internal reflection .

Critical angle and refractive index:

Critical angle and refractive index incident ray reflected ray air glass C C refracted ray sin 90 o sin C n = 1 sin C  n = C = sin 1 or 1 n ( )

Critical angle and refractive index:

Critical angle and refractive index critical angles of different materials Medium Refractive index Critical angle 1.50–1.70 30  –42  Glass Water Perspex Diamond 1.33 49  1.5 42  2.42 24 

Examples of total internal Reflection :

Examples of total internal Reflection a Prisms in optical instruments plane mirror = glass + silvered surface glass sheet silvered surface forms the image

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I 1 I 2 I 3 O object multiple reflection inside the glass multiple images formed nuisance in optical instruments

PowerPoint Presentation:

45  45  If light rays strike the inside face at an angle > 42  , glass prism behaves like a perfect mirror. 45  45  45  45 

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Prisms are used as mirrors in periscopes and binoculars.

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In single-lens reflex camera A five-sided ‘ pentaprism ’ reflects light from the mirror into the eye. film mirror

3 Examples of total internal reflection :

3 Examples of total internal reflection b Mirage On a dry road. In a desert.

3 Examples of total internal reflection :

3 Examples of total internal reflection b Mirage Hurray! On a hot day, you may see a pool of water at a distance in a desert. Sorry! What you see is just a MIRAGE .

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It happens due to total internal reflections. layers of air near the ground – hot – less dense than upper air – lower refractive indices Image of the sky

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light refracted more and more towards the horizontal Image of the sky

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When light meets a layer of air near the ground at an angle greater than C, Total internal reflections occurs. Image of the sky

3 Examples of total internal reflection :

3 Examples of total internal reflection c Light guides Light can pass through a curved perspex rod! The rays bounce inside the rod. None of them is lost to the air from the sides.

PowerPoint Presentation:

Applications 2 as a ‘light guide’ for the doctor to see inside the human body 1 optical fibre cable made up of a bundle of glass fibres glass fibre bundle of fibres

Example 5:

Example 5 A ray of light travelling in the direction EO in air enters a rectangular block. angle of incidence = 30  angle of refraction = 18  ( a) Find the refractive index n of the block. ( b) Find the critical angle C of the ray for the block. 30  18  E O

Example 5:

Example 5 ( a) By Snell’s law, n sin 18  = 1  sin 30   n = =1.62 sin 30  sin 18  ( b) C = sin 1 1 n = sin 1 1 1.62 = 38.1 

Example 5:

Example 5 ( c) If the ray is incident on surface BC , from which surface and at what angle will the ray leave the block? 30  A B C D

Example 5:

30  A B C D Example 5 The ray comes out from surface AD . The outgoing angle from normal is 60  . 32.3  57.7  32.3  60  ( c)

Q1 Which of the following…:

Q1 Which of the following… Which of the following angles is the critical angle of glass? A B D C

Q2 A horizontal light ray is…:

Q2 A horizontal light ray is … A horizontal light ray hits a prism as shown. What happens to the light ray? 45  A B C

Q3 If the breaker is 12 cm tall...:

water A small stone Q3 If the breaker is 12 cm tall... If the beaker is 12 cm tall and 8 cm wide, Can we always see the small stone below water from side A ? ( Given: Refractive index of water = 1.33)

Q3 If the breaker is 12 cm tall...:

Q3 If the breaker is 12 cm tall... Critical angle of water water A small stone 1.33 0.752 48.8 

Q3 If the breaker is 12 cm tall...:

Q3 If the breaker is 12 cm tall... For the light ray from the stone reaching side A , water A small stone 8 12 56.3 Maximum angle of incidence  on side A

Q3 If the breaker is 12 cm tall...:

Q3 If the breaker is 12 cm tall... Since  is ________ than the critical angle, ______________________ occurs and we _________ always see the stone form side A . water A small stone total internal reflection greater cannot

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