The Refraction of Light 1
Lesson 8
Fibre Optics
Light travels in straight lines but it can be
bent and directed almost anywhere.
 Optical Fibre is a thin, transparent
glass tube that can transmit light
even around corners

Fibre Optics

This is because the light in a fibre optics
tube cannot escape until it reaches the
end of the tube.
◦ Light bounces off the sides of the tube like it
was covered in mirrors.

Fibre optics systems are used to
transmit telephone and Internet
communications.
Fibre Optics
A single optical fibre can be as thin as a human hair,
yet transmit thousands of different signals at the
same time.
 This is because each signal is sent at a
different wavelength through the same
cable.
 A typical optical fibre cable can be made from
thousands of optical fibres tightly packed together

Refraction
Light normally travels in straight lines, it
when it passes from one medium to
another, such as from air to water.
 Medium – (plural:media) is a material
that is being used or is undergoing a
process.

Refraction

The bending of light rays as they pass
between two different media is called
refraction
◦ 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.
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
The Speed of Light
In the vacuum of space, where there are
very few particles, light travels at almost
300 million m/s or 3.0 × 108 m/s.
 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
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.
 The angle of refraction entering the
block is exactly reversed as the light
leaves the block

The Index of Refraction

The amount by which a transparent
medium decreases the speed of light is
indicated by a number called the index of
refraction, also called the refractive
index.

The larger the refractive index, the
more the medium decreases the
speed of light.
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. A value
of 1.00 can also be used for air in this class
as the fourth decimal place does not affect
our calculations.
 The refractive index of a medium, n, is
determined by comparing the speed of light
in the medium, v, with the speed of light in a
vacuum, c.

The Index of Refraction

Since units cancel, a refractive index value
does not have any units.
Index of refraction of Material = Speed of light in vacuum
Speed of light in medium
OR
n=c
v
The Magic Triangle
- add this to your note
C
n
v
Index of Refraction Values
Media
Vacuum
Air
CO2 gas
Water
Alcohol
Pyrex glass
Plexiglas
Table Salt
Flint glass
Sapphire
Cubic Zirconia
Diamond
Gallium phosphide
Index of refraction
1.00 (exactly)
1.0003
1.0005
1.33
1.36
1.47
1.49
1.51
1.61
1.77
2.16
2.42
3.50
Example Problem 1

The speed of light in a sample of glass is
1.91 × 108 m/s. The speed of light in a
vacuum is 3.00 × 108 m/s. What is the
refractive index of this glass?
G Speed of light in glass = 1.91 ×
108 m/s
Speed of light in vacuum = 3.00
× 108 m/s
R Refractive index n = ?
A n= c
v
S
n = 3.00 x 108 m/s
1.91 x 108
= 1.57
P
Therefore, the index of
refraction is 1.57
=
Example Problem 2

What is the speed of light in water given
that water has a reflective index of 1.33?
G Refractive index of water n = S v = 3.00 x 108 m/s
1.33
1.33
Speed of light in vacuum c =
= 2.26 × 108 m/s
3.00 × 108 m/s
R Speed of light in water v = ?
An= c
v
v=c
n
P Therefore, the speed of
light in water is 2.26 ×
108 m/s.
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.
Example
How Light Refracts

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
How Light Refracts
When light travels from air, with a low
refractive index, into water, with a higher
refractive index, it bends toward the
normal.
 When light travels from a denser (higher
refractive index) medium into a less
optically dense (lower refractive index)
medium, it bends away from the
normal.

How Light Refracts
Dispersion
The refraction of white light into
separate wavelengths, or colours.
 A diamond can appear completely
colourless and yet glitter in all colours of
the rainbow because the amount of
refraction is different for each colour.

Dispersion
The most common type of dispersion is
in the formation of a rainbow. When
sunlight passes through a raindrop, some
light is reflected.
 Some light is refracted twice, once on
entering the raindrop and once on leaving.
Both refractions cause the separation of
the white sunlight into the colours of the
rainbow

Dispersion
Practice Problems






1. The speed of light in leaded glass is 1.66 × 108.
What is the index of refraction of this type of
glass?
2. The speed of light in quartz is 2.10 × 108 m/s.
What is the index of refraction of quartz?
3. The speed of light through a material is 1.24 ×
108 m/s. What material is it?
4. What is the speed of light through alcohol?
5. What is the speed of light through gallium
phosphide?
6. What is the speed of light through sapphire
Questions
1. What is refraction? K (1)
 2. Define “index of refraction.” K (1)
 3. What refracts light more, a sapphire or a
diamond? I (1)
 4. What direction does light bend when it
travels from a denser medium to a less
dense medium? K (1)
 5. How is refraction related to dispersion?
I(1)
K(
/3) I(
/2)
