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Total internal reflection
Form 4 PhysicsForm 4 PhysicsForm 4 PhysicsForm 4 Physics
Group 14
Members: Name Student number Job
Sun Lun Chi S98006900 Leader Original scriptCheng Shu Wing S98007150 Second stage productionLee Yan Ming S98007800 IllustrationsTang Yuk Man S98006810 Additional ScriptsYau Yat Fai S98007070 Animations & SFX
November 1998
Lecture presentationfor Form 4
Contents• Reflection and refraction (revision)
• Critical angle
• Total internal reflection
• Fish eye view
glass
Reflection and refraction• Mirror reflects light
• Transparent medium refracts lightmirror
Which angle is larger?• A light ray is emerging from glass to air.
Which angle is larger, in glass or in the air ?
glassnormal
Or this?
This?AIR
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
Or this?
This?
Next
AIR
Critical angle (1)• For glass-air interface, the angle of refraction in
air > incident angle
glass
AIR
normal
Incident angle
Angle of refraction
Partial reflection
Which angle first?If the angle of incidence increases, which angle will reach 90o first, the angle of incidence or the angle of refraction?
A Angle of incidence (from glass)
B Angle of refraction (in the air)
Next
AIR
Critical angle (2)• When we increase the angle of incidence (i), the
angle of refraction (r) increases until ......
AIR
glassglass
AIR
glass
AIR
glass
i
r
normal
AIR
glass
Critical angle (3)• when the angle of refraction is 90o, the incident
angle is called the critical angle (C).
AIR
glass
r90o
normal
iC
sin 90o
sin C
Critical angle (4)• For glass-air interface, the critical angle is 42o.
• At critical angle, angle of refraction is 90o, so, by the laws of refraction,
n =sin rsin i
=1
n x sin C = 1
sin C =1n
What is the critical angle...… for water-air interface?
The refractive index of water is 1.33
sin C =1n
1 ÷ 1.33 = 0.75
sin-1 0.75 = 48.8o
=> C = 48.8o
I can do it,
so can you!
Check :
sin C = sin 48.8o = 0.75
1 ÷ 0.75 = 1.33
For glass, the refractive index is 1.5;
C = sin-1 (1÷1.5)Critical angle = 42o
Critical angle (5)
sin C =1n
For water, the refractive index is 1.33;
C = sin-1 (1÷1.33)Critical angle = 48.8o
1n
C = sin-1 ( )
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.
water
AIR
alongalong
Total internal reflection (2)• When the incident angle is larger than the
critical angle, total internal reflection will occur (at the interface).
water
AIR
Next
Total internal reflection (3)• View under water!!
water
AIR
Total internal reflection (4)• Fish-eye view
You see
total internal
reflection here
You see the sky here
through refraction
You seewater waves
here
total internalreflection here
total internalreflection here
skywaves
waves
Can you explain the ...• fish-eye view by drawing light rays?
water
Yes
Fish eye view (1)
water
Central hole, shows thesqueezed view of the sky
totally internallyreflected ray
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 2C
2C
water
What is C ?
C is critical angle
Revisit the fish eye view• Do you remember which part has refraction and
which part has total internal reflection?
total internalreflection here
sky(refraction here)
What did you learn?
• Critical angle
• 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.
1n
C = sin-1 ( )
