Refraction of Light
Section 11.1
SNC2D

Light travels in straight lines and at a
constant speed as long as it’s travelling in the
same medium.

Medium – (plural:media) is a material that is being used.
But what happens when another medium is
introduced?
 For example ... What happens if you are
riding in a golf cart on pavement, and then
you hit some mud or gravel?

- The front wheels suddenly slow down, but the back wheels keep going
and the golf cart twists around.
- This is similar to when light travels from one medium into a different
medium ... Both its speed and direction may change.
What is Refraction?

Refraction is the bending or change in
direction of light when it travels from
one medium to another
◦ Example: from air to water.


Refraction is used in designing and
building lenses.
The bending of light is due to different
media slowing light down by different
amounts. The more light slows down,
the more it is refracted.
How Light Refracts

As light enters a slower medium, the light
waves are compressed. If light strikes a
medium at an angle, the part of the light
wave that enters the medium first will
slow down first.
Refraction

This effect is commonly seen when
looking at something in the water. It
appears like the light is coming straight
to us when really it is bending.
What Causes Refraction?

Light travels at different speeds in different
media.

It is impossible for light to move at top speed
when particles get in the way. The particles in
a medium slow down the passage of the waves,
which results in light travelling more slowly
through a block of glass than through air.
The Speed of Light

Speed of light in different materials:
◦ Vacuum (empty space where there are very few
particles) = 3.00x108 m/s or 300 million m/s
◦ Air – slightly less than vacuum but use the
same speed = 3.00x108 m/s
◦ Water – 2.26x108 m/s
The Speed of Light
Example: The light ray strikes the Plexiglas at an angle.

(a) As the light enters the Plexiglas, it slows down
and refracts.

(b) When the light leaves the Plexiglas and enters
the air, it speeds up and refracts again.
The Speed of Light

Notice that light does not refract inside the
block. Light refracts only at the boundary when
it is entering or leaving a medium.
Reflection and Refraction
incident ray
NOTE: The
angles of the
refracted light
rays are usually
measured from
the normal,
drawn at 90° to
the surface
where the light
ray crosses
between the
two media.
reflected ray

So, when light travels between two media
at angles other than 90°, it bends due to
its change in speed.
RULES FOR REFRACTION
1. Incident ray, refracted ray
and the normal all lie in the
same plane. Incident ray and
refracted ray are on
opposite sides of the line
that separates the two media.

Note: Angle of refraction is
defined as the angle between
the refracted ray and the
normal.
incident
ray
Medium 1
Medium 2
RULES FOR REFRACTION
2. If the change in medium causes a decrease
in speed, then the light bends towards the
normal.
3. If the change in medium causes an increase
in speed, then the light bends away from the
normal.
Refer to diagram in handout.
The Index of Refraction

Light travels fastest in a vacuum.

The refractive index of the speed of light in a
vacuum is assigned a value of 1.00.

The larger the refractive index, the more the
medium decreases the speed of light.
The Index of Refraction

So, the index of refraction for a medium is
defined as the ratio of the speed of light in a
vacuum to the speed of light in that medium.

Mathematically, it is written as:
◦ n is the index of refraction
◦ c is the speed of light in a vacuum (in m/s)
◦ v is the speed of light in a given medium (in m/s)

n has no units (it’s a dimensionless quantity) since
m/s in c and v cancel each other out.
Index of Refraction Values
Media
Vacuum
Air
CO2 gas
Water
Ethyl Alcohol
Glycerol
Carbon disulfide
Glass (crown)
Ruby
Glass (flint)
Zircon
Diamond
Index of refraction (n)
1.00 (exactly)
1.00029
1.00045
1.3333
1.362
1.470
1.632
1.52
1.54
1.65
1.92
2.42
Example Problem 1

G
R
A
The speed of light in sodium chloride
(NaCl)1.96x108 m/s. The speed of light
in a vacuum is 3.00×108 m/s. Calculate
the index of refraction.
vNaCl = 1.96 × 108 m/s
c = 3.00 × 108 m/s
Refractive index n = ?
n= c
v
S
P
n = 3.00 x 108 m/s
1.96 x 108 m/s
n = 1.53
Therefore the index of
refraction for NaCl is 1.53
Example Problem 2

Calculate the speed of light in olive oil, if
the index of refraction of olive oil is 1.48.
G c = 3.00x108 m/s
n olive oil = 1.48
R
A
1.48
v = 2.03x108 m/s
P Therefore, the speed
v olive oil = ?
n= c
v
S v = 3.00x108 m/s
v= c
n
of light in olive oil is
2.03x108 m/s.
Dispersion




The refraction of white
light into separate
wavelengths, or colours.
Recall that white light
includes all the
wavelengths of visible light.
The order of the colours
of light in a spectrum are:
red, orange, yellow, green,
blue, indigo, violet [ROY G
BIV].
Note that blue light bends
more than red. So blue
light travels slower than
red light.
Homework
Read pg. 449-456
 Answer # 1-4 on pg. 455
 Answer # 1-8 on pg. 456
