PHYA2 - Optics

Refraction is the change of direction that occurs when light enters a medium non-normally  across a boundary between 2 transparent substances

The light ray bends:

• Away from the normal if it enters a less dense medium (e.g. water to air) 
• Towards the normal if it enters a more dense medium (e.g. glass to air)

The refractive index of air is the ratio of the speed of light in a vacuum to the speed of light in that material

When an incident ray is at the critical angle and enters a less dense medium, the angle of refraction = 90 degrees (refraction along the boundary)

• TIR only occurs if the angle of incidence is greater than the critical angle
• The material the light is going into has to have a lower refractive index for the light to bend away from the normal and towards the boundary
• Partial internal reflection always occurs when the angle of incidence is less than or equal to the critical angle 

From more dense medium to air: 

• Diamond has a high refractive index therefore the critical angle is low
• It seperates white light more than any other substance
• The light may be totally internally refracted many times before it leaves

Optical fibres use total internal reflection to send data across a tube

• Each time a light ray reaches the fibre boundary it is totally internally reflected
• The incident ray of the is always greater than the critical angle of the fibre
• Cladding increases the critical angle, protects the glass core and prevents leakage of light
• The core should be narrow to prevent multipath dispersion