TOTAL INTERNAL REFLECTION
16 APRIL 2013
Lesson Description
In this lesson, we:



Discuss and define Snell’s Law
Verify Snell’s Law
Discuss and define total internal reflection
Key Concepts
Snell’s Law
As light moves from medium to another it is refracted. This refraction occurs because the speed of
the light changes and causes the light to change direction (if it hits the boundary at an angle greater
o
than 0 )
To describe the extent to which the speed of light in a medium differs from that in a vacuum we use a
quantity called the refractive index.
Refractive index: The refractive index n of a material is the ratio of the speed c of light in a vacuum
to the speed v of light in the material.
Refractive index of some common materials
Substance
Air
Water
Perspex
Glass
Diamond
Refractive index
1,0
1,33
1,5
About 1,5
2,4
Note: The speed of light in a vacuum and air is significantly close
enough for the refractive index of air to be the same as the
refractive index for a vacuum, i.e. I,0.
When light travels from a medium where the refractive index is
smaller (i.e. air) into a medium where it is larger (i.e) glass, the light
is refracted towards the normal. The amount of refraction that
occurs is dependent on the refractive index of the materials
involved and the angle of incidence. The relation between theses
quantities is known as Snell’s law of refraction.
Met
Snell’s Law of Refraction: When light travels from a material with refractive index n1 into a material
with refractive index n2, the refracted ray, the incident ray and the normal to the interface between the
materials all lie in the same plane. The angle of refraction, θr, is related to the angle of incidence, θi,
by:
Demonstration

Place a glass block on a sheet of paper, and mark its position.

Allow a ray of light to hit one side of the block at an angle of about 30 .

Look on the other side of the block to see where the ray of light is emerging.

Mark the directions of the incident and emergent rays to and from the glass block.

Join the two rays to show the direction of the ray through the block.

Measure the angle of incidence and angle of refraction. Compare the angle of refraction to the
expected angle of refraction if calculated using Snell’s Law of Refraction.
o
Questions
Question 1
Light is passed from air into a rectangular glass prism.
o
a.) What angle of incidence would give an angle of refraction of 10 ?
o
b.) What angle of refraction would you get for an angle of incidence of 36 ?
Question 2
o
A light ray strikes an air/water surface at an angle of 46 with respect to the normal. The refractive
index for water is 1,33. Find the angle of refraction when the direction of the ray is
a.) from air to water
b.) from water to air
Key Concepts
Total Internal Reflection
Consider a ray of light moving from water to air. It will be refracted away from the normal
o
There is a certain angle at which the refracted ray is refracted at 90 , i.e. along the water surface. In
this case we say that the angle of incidence is equal to the critical angle.
Any ray of light that is incident at greater than the critical angle, cannot be refracted into the air. It is
refracted back into the water. This phenomenon is known as total internal refraction.
The conditions for total internal refraction to take place:
1. Light ray must move from greater optical density to a medium with lower optical density
2. Angle of incidence must be greater than the critical angle
Met
Application of Total Internal Refraction
o
Since the critical angle of glass is 42 , any light travelling in glass that hits the glass/air interface at
o
more than 42 will be reflected back into the glass.
o
Consider a 45 prism
o
A ray of light hitting the side at 90 passes straight through, and hits
o
the hypotenuse side at 45
45o
o
45o o
45
The law of reflection tells us that the ray will now be reflected at 45 .
o
The light hits the second side at 90 and passes straight out. We have
o
therefore been able to change the direction of the light ray through 90
The principle of Total Internal Reflection applies to:
1. The Periscope
2. Binoculars
Met
3. Fibre Optics
Uses of Fibre Optics: Telecommunications
Applications are widespread, ranging from global networks to desktop computers. These involve the
transmission of voice, data, or video over distances of less than a metre to hundreds of kilometres,
using one of a few standard fibre designs in one of several cable designs.
Optical fibres carry telephone service across their nationwide networks.
Optical fibre is used extensively for the transmission of data. Multinational firms need secure reliable
systems to transfer data and financial information between buildings to computers and to transfer data
around the world.
Cable television companies also use fibre for delivery of digital video and data services.
Links
1. Snell’s Law of Refraction: http://www.youtube.com/watch?v=yfawFJCRDSE
2. Snell’s Law example: http://www.youtube.com/watch?v=UXLb0AISxrs
3. Critical angle and total internal reflection: http://www.youtube.com/watch?v=CF7CJb8X
Met