Total internal reflection Form 4 Physics Contents Reflection

Total internal reflection Form 4 Physics

Contents • • Reflection and refraction (revision) Critical angle Total internal reflection Fish eye view

Reflection and refraction • Mirror reflects light mirror • Transparent medium refracts light glass

Which angle is larger? • A light ray is emerging from glass to air. Which angle is larger, in glass or in the air ? normal glass Or this? AIR This?

Which angle is larger? • A light ray is emerging from glass to air. Which angle is larger, in glass or in the air ? normal glass Or this? AIR This? Next

Critical angle (1) • For glass-air interface, the angle of refraction in air > incident angle glass normal Partial reflection Incident angle Angle of refraction AIR

Which angle first? If the angle of incidence increases, which angle will reach 90 o first, the angle of incidence or the angle of refraction? A Angle of incidence (from glass) B Angle of refraction (in the air) Next

Critical angle (2) • When we increase the angle of incidence (i), the angle of refraction (r) increases until. . . normal glass i r AIR

Critical angle (3) • when the angle of refraction is 90 o, the incident angle is called the critical angle (C). normal glass i. C 90 or AIR

Critical angle (4) • For glass-air interface, the critical angle is 42 o. • At critical angle, angle of refraction is 90 o, so, by the laws of refraction, sin r sin 190 o n= = sin i sin C n x sin C = 1 1 sin C = n

What is the critical angle. . . … for water-air interface? The refractive index of water is 1. 33 1 ÷ 1. 33 = 0. 75 sin-1 0. 75 = 48. 8 o => C = 48. 8 o Check : sin C = sin 48. 8 o = 0. 75 1 ÷ 0. 75 = 1. 33 1 sin C = n I can do it, so can you!

Critical angle (5) For glass, the refractive index is 1. 5; C = sin-1 (1÷ 1. 5) Critical angle = 42 o For water, the refractive index is 1. 33; C = sin-1 (1÷ 1. 33) Critical angle = 48. 8 o -1 Csin = sin C= 11 ( ) nn

What if. . . … the angle of incidence (from glass) is larger than the critical angle? A The angle of refraction will become smaller again. B There will be no refracted ray. C The light ray will be reflected the same way back. Next

Total internal reflection (1) • When the light ray (from water) is emerging at the critical angle, the refracted ray will be along the interface. AIR water

Total internal reflection (2) • When the incident angle is larger than the critical angle, total internal reflection will occur (at the interface). AIR water Next

Total internal reflection (3) • View under water!! AIR water

Total internal reflection (4) • Fish-eye view sky waves see Youthe seesky here You see water waves total internal through refraction here reflection waves total internal reflection here

Can you explain the. . . • fish-eye view by drawing light rays? water Yes

Fish eye view (1) Central hole, shows the squeezed view of the sky otally internally reflected ray water

Fish eye view (2) • When a diver or fish looks upward under water, part of the water surface looks like a mirror, except the central hole. • So, the light entering the water from above the surface is squeezed into a cone of angle 2 C 2 C What is C ? critical angle water C is

Revisit the fish eye view • Do you remember which part has refraction and which part has total internal reflection? sky(refraction here) total internal reflection here

What did you learn? • Critical angle C= sin-1 1 ( ) n • Total internal reflection occurs when a light ray tries to escape from water (or glass) to air at an angle of incidence larger than the critical angle at the interface. • Fish eye view is an example of total internal reflection and refraction.