1. ____
_______
_________

2. Total Internal Reflection of Light – When Refraction Becomes Reflection!!!
When light travels from a more dense medium into a less dense medium (eg glass to air), refraction and reflection of light can occur at the boundary.
What happens depends on the angle of incidence and the critical angle, as the following three diagrams show.
3. Angle of Incidence is (BIG) greater than the critical angle I
2. Angle of Incidence is equal to the critical angle
1. Angle of Incidence I is (SMALL) less than the critical angle
glass
normal
light ray
Critical angle
Most of the light is refracted at the boundary.
All light is internally reflected at the boundary. This is Total Internal Reflection (TIR). R
I = R
The refracted ray travels along the boundary.
[critical angle is about 420 for glass-air] R R

3. Uses of TIR - Optical Fibres
An optical fibre is a long, flexible rod, made of glass or transparent plastic, which is very narrow.
Light sent in one end of an optical fibre travels through to the other end by undergoing total internal reflection (TIR) along its length.
Refraction
Refraction (angle of incidence is less than critical angle)
glass
air
TIR (angle of incidence is greater than critical angle)

4. Uses of Optical Fibres 1. Telecommunications 2. Endoscopes
The light travelling down optical fibres can be encoded and used to carry information. Optical fibres have several advantages over similar thickness cables carrying electrical signals:
able to carry more information
less weakening of the signal as it travels along
more secure and reliable
2. Endoscopes
These are devices used to view inside the human body. They consist of many fibres, half of which transmit light to the body part being viewed, and half transmit reflected light back to form and image.
Spy cameras work in a similar way.

5. Other Uses of TIR
450
Reflecting Prisms to reflect through 900 or 1800 – better than mirrors since no light is absorbed and a brighter image is seen.
These are used in:
Periscopes Binoculars Bicycle Reflectors