Total internal reflection

Group 14
Members:
Name Student number Job
Sun Lun Chi
Cheng Shu Wing
Lee Yan Ming
Tang Yuk Man
Yau Yat Fai
S98006900
S98007150
Leader
Original script
Second stage production
S98007800 Illustrations
S98006810 Additional
Scripts
S98007070 Animations
& SFX
November 1998

Lecture presentation for Form 4

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

Reflection and refraction
• Mirror reflects light
• Transparent medium refracts light mirror 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?
This?
AIR
Next

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

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 i r

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

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, n = sin r sin i n x sin
C
= 1
= sin 90 o sin
C sin
C
=
1 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
sin
C
=
1 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
C sin
C
=
1 1
( ) n n

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!!
water
AIR

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

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

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

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.