Download - Refraction Week2
3 Refraction of light
3 Refraction of light
3 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.
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
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.
Total internal reflection and critical angle
strong refracted ray
strong reflected ray
C
The angle of incidence is larger than the critical angel.
Total internal reflection and critical angle
very strongreflected ray
a-d. Light emitted in the water at angles below the critical angle is partly refracted and partly reflected at the surface.
Total Internal Reflection
a-d. Light emitted in the water at angles below the critical angle is partly refracted and partly reflected at the surface. e. At the critical angle, the emerging beam skims the surface.
Total Internal Reflection
a-d. Light emitted in the water at angles below the critical angle is partly refracted and partly reflected at the surface. e. At the critical angle, the emerging beam skims the surface. f. Past the critical angle, there is total internal reflection.
Total Internal Reflection
a. The apparent depth of the glass block is less than the real depth.
b. The fish appears to be nearer than it actually is.
Refraction of Light
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.
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
Reflection and Refraction
When you watch the sun set, you see the sun for several minutes after it has really sunk below the horizon.
Since the density of the atmosphere changes gradually, refracted rays bend gradually to produce a curved path.
The same thing occurs at sunrise, so our daytimes are about 5 minutes longer because of atmospheric refraction.
Atmospheric Refraction
Reflection and Refraction
When the sun is near the horizon, the rays from the lower edge are bent more than the rays from the upper edge. This produces a shortening of the vertical diameter and makes the sun look elliptical instead of round.Atmospheric refraction produces a “pumpkin” sun.
Atmospheric Refraction
Reflection and Refraction
Reflection and Refraction