Refraction
Refraction – The bending (change in direction) of light when
it travels from one medium to another. Eg. From air into
water. Refraction occurs because the speed of light
changes depending on the medium. *Note that nothing
exceeds the speed of light in a vacuum.
Rules:
1. The incident ray, refracted ray, and normal are on the
same plane. The rays are on opposite sides of the
boundary.
2. Light bends toward the normal when the speed of light
in the second medium is slower than the first. It bends
away if the speed of light is faster in the second.
The Index of Refraction
Recall that the speed of light varies depending on the
medium through which the light is travelling, and this is what
causes refraction.
Index of refection (n) is the ratio of the speed of light in
vacuum to the speed of light in a particular medium.
c
n =
______
v
Where “n” is the index of refraction, “c” is the speed of light
in vacuum
(3.00 x 108 m/s), and “v” is the speed of light in a given
medium.
Critical Angle and Total Internal Reflection
Critical Angle is the angle of incidence that results in a 90°
angle of refraction.
- If the angle of incidence is greater than the critical
angle, then total internal reflection occurs
Total internal reflection is the reason why diamonds and other
gems sparkle when moved under lights.
Car and bicycle reflectors are designed to reflect light by total internal
reflection.
Fibre optics use total internal reflection: hair-like strands of glass
reflect light energy internally along their entire length. Uses of fibre
optics:
 perfect images can be reflected internally along these fibres for
several metres
 surgeons use endoscopy to shine a light down the fibres to
observe tissues in the lungs, stomach and intestines
 radio, TV, and telephone signals can be transmitted more
quickly and more efficiently
 tens of thousands of phone calls can be transmitted in either
direction at the same time on a single optical fibre
 the automobile industry uses optical fibres to transmit light to
the instrument panel in cars
Phenomena Related to Refraction
Apparent Depth
 apparent depth is an optical illusion
 this is what makes fish in water seem closer to the surface
than they actually are
 light from the fish reaches your eyes, but the light bends away
from the normal when it moves from water to air [because
speed of light is faster in air]
 your brain then projects the rays BACKWARDS into the water
in a straight line to create a virtual image in the water
 the virtual image is higher than the actual fish, resulting in the
fish appearing to be more shallow than it actually is
Shimmering – such as a shimmering image of the Moon on the
surface of a lake at
night
 shimmering is caused by light being refracted as it passes
through air of different temperatures
 at night, the air just above the water is warmer than the air
further away from the water’s surface
 Moonlight passes through layers of air with different
temperatures
 In the coldest air layer, light travels more slowly, so light rays
will bend towards the normal
 In the warmer layers, light travels more quickly, so light rays will
bend away from the normal
 Eventually, total internal reflection occurs in the lowest warm air
layer, resulting in many virtual images being formed of the
Moon on the lake’s surface
Mirages - such as a motorist seeing a pool of water on the pavement
as they drive
along the highway
 A mirage is a virtual image that forms as a result of refraction
and total internal reflection in the Earth’s atmosphere
 Mirages occur when light travels from cool air into warmer air
 The index of refraction for air decreases as the air gets warmer,
so the speed of light increases
 Light therefore bends farther away from the normal as the air
temperature increases
 Eventually, total internal reflection occurs in the warmest layers
of air
 The light ray then travels upwards from the hottest layer to the
cooler layer above and is gradually refracted towards the
normal as the air temperature decreases
 This ray of light eventually enters the driver’s eyes. The
motorist who sees this curved light forms a virtual image on the
highway
 Since the human brain perceives light to travel in a straight line,
the motorist projects the image of the sky onto the highway
Mirage - the ‘pool of water’ is actually a virtual
image of the sky on the highway.
Rainbows
 A rainbow is formed by the refraction and total internal
reflection of light and the resulting dispersion of the light by
spherical water droplets in the sky
 You can only see a rainbow when the Sun is behind you
 3 steps are involved in the production of a rainbow:
1. Light refracts as it enters a raindrop [going from air to
water] which results in dispersion (separation of white
light into its spectrum)
2. Partial internal reflection occurs when this light hits the
back of the raindrop
3. Light refracts as it exits the raindrop [going from water to
air]. THIS IS THE LIGHT THAT YOU SEE IF THE SUN
IS BEHIND YOU. Your brain perceives this light as a
rainbow.