12.1 What is Refraction
pg. 515
Light travels in straight lines through air.
What happens to light when it travels from one material into
another?
Bending Light
The light traveling from an object in water to your eyes appears to
travel in a straight line. Yet if you try to reach the coin in the water
you miss it. The light traveling from the water travels from one
medium into another, this will cause the light to either slow down
or speed up, causing to bend.
When light slows down or speeds up, it is called refraction of light.
Refraction is the bending or changing in direction of light when it travels
from one medium into another.
Figure 1 – the refraction of light
going from air into water
What Causes Refraction


When light strikes a translucent or transparent material,
it bends, or refracts.
The refraction of light is due to its speed changing as it
passes from one material into another.
Figure 3: A wagon changes direction when traveling at an angle from the
pavement onto sand because on front wheel slows down while the other wheels
continue moving at a higher speed.


Depending on the material, light either speeds up or
slows down.
Light cannot travel any faster than 3.0 x108 m/s, which
is the speed of light in a vacuum.
How fast is the Speed of Light?
The speed of light in water is different from the speed of light in
air.
Speed of light in Air is 3.0 x108 m/s
Speed of light in Water is 2.26 x108 m/s
Speed of light in Acrylic is 1.76 x108 m/s
The Rules for Refraction
The speed of light changes depending on the medium through
which the light is traveling.
The Angle of Refraction is the angle between the refracted ray and
the normal.
There are three predictable results when light refracts:
1. The incident ray, the refracted ray, and the normal all lie in
the same plane.
2. Light bends toward the normal if it travels more slowly in the
second medium than in the first.
3. Light bends away from the normal if it travels faster in the
second medium than in the first.
The Bent Spoon
The light traveling from the spoon to your eye must travel through
water (2.26 x108 m/s) to air (3.0 x108 m/s). The speed of light
leaving the spoon increase when it enters air at the boundary
between the water and air.
The angle of incidence is measured from the normal to the
incident ray and the angle of refraction is measured from the
normal and the refracted ray. As the light enters air it speeds up
and bends away from the normal.
The brain perceives the light is traveling a straight line and does
not account for refraction.
Partial Reflection and Refraction
Refraction is often accompanied by reflection. Some of the light
that strikes water is reflected, while some light is refracted.
Windows exhibit the same property. The property creates a
mirrored surface.
Check Your Learning, questions 1 – 7, page 519
12.4: The Index of Refraction
pg. 524
French Physicist Jean Foucault made the first measurement of the
speed of light in a medium (other than a vacuum or air) in 1862.
The speed of light in air is 3.00 x 108 m/s and the speed of light in
water is 2.26 x 108 m/s.
The speed of light is different in different mediums.
The change in speed of light at the boundary of the mediums
causes Refraction.
Refraction: the bending of light or change in direction of light
when it travels from one medium into another.
These changes are distinct optical properties.
Index of Refraction: for a medium is the ratio of the speed of light
in a vacuum to the speed of light in that medium.
Index of Refraction: the ratio of the speed of light in a vacuum to
the speed of light in a medium, n = c/v; this value is also equal to
the ratio of the sine of the angle of incidence ray in a vacuum to
the sine of the angle of refraction ray in a medium,
n=
n = Index of Refraction
equation:
n = c
c = speed of Light in a Vacuum
v
v = speed of light in a Medium
or
equation:
n = sin ۧ i
sin ۧ R
ۧ i = angle of Incidence
ۧ R = angle of Refraction
Problem #1: (GRASP)
Given: Speed of Light in Air: 3.00 x 108 m/s
Speed of Light in Sodium Chloride (NaCl)
1.96 x 108 m/s
Required: Determine the Index of Refraction (n)
Analysis: Formula
n = c
v
Solution:
n = c
v
Sub:
n = 3.00 x 108
1.96 x 108
m/s
m/s
n = 1.53
Paraphrase: The Index of Refraction is 1.53 for
Sodium chloride (NaCl)
Problem #2:
Given: Speed of light 3.00 x 108 m/s
Index of Refraction is 1.48 for Olive Oil
Required:Determine the Speed of Light through the Olive Oil.
Analysis: Formula
n = c
v
Solution: n = c
v
Sub:
n = c
v
1.48 = 3.00 x 108
v
m/s
v x 1.48 = 3.00 x 108 x v
v
m/s
v x 1.48 = 3.00 x 108
1.48
1.48
m/s
v
= 3.00 x 108
1.48
v
= 2.03 x 108 m/s
m/s
Paraphrase: the speed of light through the Olive Oil is
2.03 x 108 m/s
Check Your Learning: questions 1 – 10, pg. 525
12.5: Total Internal Reflection
pg. 526
As light enters a medium, some is reflected and is refracted.
Light moves further from the normal when it speeds up.
The Angle of incidence is less than the Angle of refraction.
Light moves closer to the normal when it slows down.
The Angle of incidence is greater than the Angle of refraction.
Eventually the angle of refraction will become 900. Therefore the
angle of incidence has reached a point called the Critical Angle.
Critical Angle: is the angle of incidence that results in an angle of
refraction of 900.
If the Critical Angle is passed, the refracted ray will no longer
exit the medium, but will reflect back into the medium from which
it came, the refracted ray disappears.
This event is called Total Internal Reflection.
Total Internal Reflection: the situation when the angle of
incidence is greater than the critical angle.
Total Internal Reflection occurs only if 2 conditions are met;
1. Light is traveling more slowly in the first medium than in
the second.
2. The angle of incidence is large enough that no refraction
occurs in the second medium. The ray is reflected back into
the medium.
Water has a critical angle of 48.80.
If the angle of incidence is greater than 48.80, a total internal
reflection will occur.
Figure 3: Ray 1 is refracted as it passes from water into air. Ray 2 has an angle of
refraction of 900; the angle incidence is equal to the critical angle. Ray 3 is reflected
internally back into water and does not go into air. Note that in (a) and (b), some light
is reflected internally but not as strongly as in (c).
Diamonds Are Forever
Diamonds sparkle due to the cut of the diamond faces.
Diamonds have a high index of refraction, n = 2.42
Diamonds also have a critical angle of 24.40.
Therefore a lot of light undergoes Total Internal Reflection.
Figure 4: (a) Many light rays undergo two total internal reflections inside a diamond
(b) this is what makes a diamond “sparkle”. pg. 527
Fibre Optics
Technology which uses light to transmit information is called
Fibre Optics.
Light travels in a glass cable and it is not permitted to escape.
The cable has a small critical angle, most light entering has a
angle of incidence greater than the critical angle, therefore the
light undergoes Total Internal Reflection.
Figure 5: (a) A laser beam undergoes total reflection in a Lucite rod. (b) In close-up,
you can see the point at which total internal reflection occurs. pg. 528
Triangular Prism
A Triangular Prism exhibits total internal reflection. The critical
angle for glass is 41.10.
If the angle of incidence is greater than 41.10, total internal
reflection occurs, and 100% of the light is reflected.
The Emergent Ray can be either 900 or 1800 to the Incident Ray,
this depends on the prism.
Retro – reflectors and Prisms
Retro-reflector is an optical device in which the emergent ray is parallel to
the incident ray.
The incident ray is returned back in exactly the same direction from
which it came.
retroreflectors
• bike reflectors •
reflective strips • light can return along the
on clothing and same path as the incident
helmets • road
light
signs
Check Your Learning, questions 1 – 9, pg. 531
12.7: Phenomena Related to Refraction
pg. 535
Apparent Depth
- The depth that an object appears to at due to the refraction of
light in a transparent medium.
- Our brain perceives light rays travel in straight lines.
- Looking a figure 2. The pencil in the water is perceived to be
in a specific location, based on our brain‟s understanding of
light travels in a straight line, is actually a „Virtual Image”.
Fig. 2: Refraction causes the pencil to appear closer to the surface than it actually is.
pg. 535
- The Virtual Image is higher then the Actual Image.
- This is an optical illusion
- When looking at a fish in the water it appears closer to the
surface then where it is really located, deeper.
Figure 3: The Illusion of apparent depth. Pg. 535
The Flatten Sun
As the sun sets and moves closer to the horizon, the sun appears
flat. The light from the bottom of the sun is refracted more then the
light generated from the top of sun.
The phenomena created, because there is a change (difference) in
the angle of incidence between the top and bottom of the sun. This
difference is created because of the density of air near the Earth‟s
surface.
Figure 5: the flattening of the Sun is the result of refraction in Earth‟s atmosphere.
pg. 536
Water on Pavement – The Mirage
On some days you can notice what appears to be a pool of water in
front of you as you drive along a road. It always appears in front of
you but you never seem to catch up to it.
This pool of water is called a mirage. The mirage is created when
light travels from cooler air into warmer air, and the index of
refraction decreases as the air gets warmer. The result is the light
bends further away from the normal. As the light rays move closer
to a 900 angle, total internal reflection occurs.
The virtual image created is actually an image of the sky on the
highway.
Figure 6b: There appears to a pool of water on the highway. This illusion is caused by
the refraction and reflection of light as it goes through air of different temperatures.
pg. 537
Shimmering
The image of the Moon on the water is created when light travels
from the Moon to the surface of the water. The light travels
through cooler air to warmer air near the surface. The light travels
more slowly in the cooler air and begins to speed up in the warmer
air. This causes different refraction angles, bending further from
the normal. Eventually total internal reflection occurs, resulting in
multiple virtual images on the water‟s surface.
Figure 7: Shimmering on a lake is caused by light traveling at slightly different speeds
through air layers of different temperatures. pg. 537
The Rainbow
Dispersion – is the separation of white light into its constituent
colours.
Isaac Newton used a triangular prism to separate white light into a
continuous sequence of colours. The dispersion of light occurs
because visible light travels at a slightly different speed when goes
through the prism.
Refraction occurs, with violet bending more toward the normal and
the red light the least.
The rainbow is an optical phenomenon that is produced by water
droplets in the Earth‟s atmosphere. The rainbow can only be seen
when the sun is located behind you. The light from the sun passes
into the water droplet, and is refracted, then internal reflection
occurs and finally leaves the droplet as it is refracted again.
Figure 10: the rainbow is caused by a combination of dispersion and partial internal
reflection in water droplets in the atmosphere. Millions of raindrops are necessary to
produce a rainbow. pg 538
Check Your Learning, questions 1 – 7, pg. 539