5.3 Total internal reflection Flashcards
Define critical angle
the angle of incidence of a light ray must exceed the critical angle for total internal reflection to occur.
Define total internal reflection
a light ray travelling in a substance is totally internal reflected at a boundary with a substance of lower reactive index, if the angle of incidence is greater than a certain value known as the critical angle
What are the 2 conditions required for TIR?
- The incident substance has a larger refractive index than the other substance
- The angle of incidence exceeds the critical angle
Why do diamonds sparkle when white light is directed at them?
Diamond has a very high refractive index of 2.417 so it separates the colours more than any other substance does. Being highly refractive, diamond has a critical angle of only 24 degrees so light ray in a diamond may be totally internally reflected many times before it emerges which means its colours spread out more and more.
Define optical fibres
a thin flexible transparent fibre used to carry light pulses from one end to the other
What is an endoscope?
optical fibre device used to see inside cavities
What is the purpose of communications optical fibres?
They allow pulses of light that enter at one end, from a transmitter, to reach a receiver at the other end.
What is the structure of an optical fibre and why?
- glass core (transparent) to minimise absorption of light
- core surrounded by a layer of cladding of LOWER refractive index to REDUCE light loss from the core
What might light loss in an optical fibre result in?
It would reduce the amplitude of the pulses
When does partial internal reflection occur?
At a boundary when the angle of incidence is less than or equal to the critical angle.
What would happen if 2 fibres were in direct contact with no cladding?
Light would cross from one fibre to the other.
Signals would not be secure as they would reach the wrong destination
Define modal (multi path) dispersion
the lengthening of a light pulse as it travels along an optical fibre, due to rays that undergo less total internal reflection
How do you reduce modal dispersion?
Make the core narrow
Define material dispersion
Different wavelengths of light travel at different speeds depending on the refractive index of the material used in the fibre core.
The difference in speed (of different coloured light) would cause the white light pulses in optical fibres to become longer as one falls behind the other.
How do you reduce material dispersion?
Use monochromatic light
