# Total internal reflection and critical angle ```3
Refraction of light
3
Refraction of light
3
Refraction of light
3
Refraction of light
3.1
Refraction of light
3.2
Laws of refraction
3.3
Snell’s law and refractive index
3.4
Refraction through a block
3.5
Refraction through a prism
3.6
Real depth and apparent depth
3.7
Total internal reflection and critical angle
3
Refraction of light
3.8
Total internal reflection in prisms
3.9
Optical fibres
3.10
Formation of a mirage
Summary
3.1
Refraction of light
When light travels from one medium to another, it is
bent or refracted. This is because light travels at
different speeds in different media.
Light travels faster in optically less dense media and
slower in optically dense media.
3.1
Refraction of light
The car hits the grass at an oblique angle. As
the car changes speed, it changes direction.
3.1
Refraction of light
The car hits the grass at right angles.
There is no change of direction.
3.1
Refraction of light
incident ray
normal
angle of
incidence i
air
glass
angle of
refraction r
refracted ray
Describing refraction.
3.2
Laws of refraction
1 Rays of light travelling from air into glass are
refracted towards the normal.
2 Rays of light travelling from glass into air are
refracted away from the normal.
We can conclude that light bends towards the
normal in optically denser material.
Experiment 3.1
Laws of refraction
3.2
Laws of refraction
The laws of refraction state that
1 The incident ray, the refracted ray and the normal
are in the same plane.
2 The ratio of the sine of the angle of incidence to
the sine of the angle of refraction is a constant,
this is called Snell's law.
sin i
sin r
=
constant
3.2
Laws of refraction
A graph of sin i against sin r.
3.3
Snell’s law and refractive index
This ratio of sin i to sin r for light rays passing from
air to a medium is called the refractive index of
that medium or nm. That is
nm =
sin i
=
sin r
sin
a
sin
m
where a is the angle of incidence in air and m is the
angle of refraction in the medium.
3.3
Snell’s law and refractive index
Material
Refractive index n
Vacuum
Air
Water
Perspex
Glass
Diamond
1.00
1.0003 (at 20&ordm;C)
1.33
1.50
1.50 – 1.70
2.42
Refractive indexes of some materials.
3.4
Refraction through a block
A
incident ray
D
a
b
c
d
emergent ray
B
C
lateral
displacement
Refraction by a rectangular glass block.
3.5
Refraction through a prism
A
angle of
deviation
emergent
ray
incident ray
B
C
Deviation of light by a prism.
3.6
Real depth and apparent depth
The apparent depth is less than the real depth
because rays of light are refracted from the normal
as they leave the water.
The light rays appear to come from a point I which
is a virtual image.
Experiment 3.2
Apparent depth
3.6
Real depth and apparent depth
apparent
depth
virtual
image
I
The real and apparent depth of a swimmer.
real
depth
3.7
Total internal reflection and critical angle
The inside of a glass block can act like a plane mirror.
and the critical angle C in the following experiment.
Experiment 3.3
Critical angle
3.7
Total internal reflection and critical angle
very strong
refracted ray
semi-circular
glass block
ray box
very weak
reflected ray
The angle of incidence is small.
3.7
Total internal reflection and critical angle
strong refracted ray
C
strong reflected ray
The angle of incidence is equal to the critical angle C.
3.7
Total internal reflection and critical angle
very strong
reflected ray
The angle of incidence is larger than the critical angel.
3.8
Total internal reflection in prisms
Prismatic periscope.
Experiment 3.4
Construction of a prismatic periscope
3.8
Total internal reflection in prisms
45o
45o
Two prisms being used in binoculars. Notice
that the light rays are turned through 180 &ordm;
after two internal reflections.
3.9
Optical fibres
single glass fibre
emergent light
beam
light beam
bundle of fibres
A bundle of glass fibres. Notice the total internal reflection
in the fibre.
3.10 Formation of a mirage
Sometimes, on a hot day, we see mirages. For
example, a driver may see what looks like a large
pool of water on the road. In fact, the road is dry.
3.10 Formation of a mirage
cool air
warm air
total internal reflection here
light from the sky
looks like a pool of water
hot air
The road seems wet on a hot day. This is called a mirage.
3.10 Formation of a mirage
A mirage is formed because there are hot layers of air
near the hot road and a cooler and denser layer above
it. Cool air has a greater refractive index than hot air.
Summary
•
Total internal reflection and critical angle
9 Total internal reflection takes place when
•
light rays are travelling from an optically
denser region to a less dense region, and
•
the angle of incidence is greater than the
critical angle.
1
10 The refractive index =
sin C
or
1
C = sin-1 ( n )
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