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
When light travels from one medium to another, it is
bent or refracted. This is because light travels at
different speeds in different media.
3.1 Refraction of light
Light travels faster in optically less dense media and
slower in optically dense media.
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
Describing refraction.
3.1 Refraction of light
air
glass
incident ray normal
refracted ray
(
angle ofincidence i
(angle ofrefraction r
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.
3.2 Laws of refraction
Experiment 3.1 Laws of refraction
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.
The laws of refraction state that
3.2 Laws of refraction
sin i
sin r= constant
A graph of sin i against sin r.
3.2 Laws of refraction
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
3.3 Snell’s law and refractive index
nm =sin i
sin r=
sin
sin ma
where a is the angle of incidence in air and m is the
angle of refraction in the medium.
Material Refractive index n
Vacuum
Air
Water
Perspex
Glass
Diamond
1.00
1.0003 ((at 20ºat 20ºC)C)
1.33
1.50
1.50 – 1.70
2.42
Refractive indexes of some materials.
3.3 Snell’s law and refractive index
Refraction by a rectangular glass block.
3.4 Refraction through a block
A D
B C
(
(
(
(
incident ray
abc
d
emergent ray
lateraldisplacement
Deviation of light by a prism.
3.5 Refraction through a prism
B C
A
incident ray emergentray
angle ofdeviation
The apparent depth is less than the real depth
because rays of light are refracted from the normal
as they leave the water.
3.6 Real depth and apparent depth
The light rays appear to come from a point I which
is a virtual image.
Experiment 3.2 Apparent depth
The real and apparent depth of a swimmer.
3.6 Real depth and apparent depth
real depthreal depth
I
apparent depth
virtual image
The inside of a glass block can act like a plane mirror.
We can learn more about total internal reflection
and the critical angle C in the following experiment.
3.7 Total internal reflection and critical angle
Experiment 3.3 Critical angle
The angle of incidence is small.
3.7 Total internal reflection and critical angle
ray box
semi-circularglass block
very strongrefracted ray
very weakreflected ray
The angle of incidence is equal to the critical angle C.
3.7 Total internal reflection and critical angle
strong refracted ray
strong reflected ray
C
The angle of incidence is larger than the critical angel.
3.7 Total internal reflection and critical angle
very strongreflected ray
Prismatic periscope.
3.8 Total internal reflection in prisms
Experiment 3.4 Construction of a prismatic periscope
Two prisms being used in binoculars. Notice that the light rays are turned through 180 º after two internal reflections.
3.8 Total internal reflection in prisms
45o 45o
A bundle of glass fibres. Notice the total internal reflection in the fibre.
3.9 Optical fibres
emergent light beam
single glass fibre
bundle of fibres
light beam
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
The road seems wet on a hot day. This is called a mirage.
3.10 Formation of a mirage
cool air
warm air
hot airtotal internal reflection herelooks like a pool of waterlight from the sky
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.
3.10 Formation of a mirage
Summary
• Total internal reflection and critical angle
9 Total internal reflection takes place when
• the angle of incidence is greater than the
critical angle.
• light rays are travelling from an optically
denser region to a less dense region, and
110 The refractive index =
sin C
or C = sin-1 ( )1n