Refraction Flashcards
Refraction
Where a wave changes direction as it enters a different medium as a result of it changing speed
Why does light refract
Different materials have different optical density
How is optical density measured?
By the refractive index
When light enters a more optically dense material…
…it refracts towards the normal
When light enters a less optically dense material…
…it refracts away from the normal
When a light hits a surface normally…
…there is no refraction
Conditions for total internal refraction to occur
- Light moving towards a material with a lower refractive index
- The light hits the boundary at / above the critical angle
Critical angle
The angle of incidence that results in an angle of refraction of 90° (when traveling into a less optically dense material)
What is the function of optical fibres?
Transmiting light over long distances
Structure of a fibre optic cable
Core - Cladding - Protective layer
Advantages of using optical fibres (over copper cables)
- Less energy loss per meter (longer distances between signal being boosted)
- High frequency of light allows more data transfer per seccond
- No electrical interference (fibres can be bundled together)
- More secure (cannot be detected/tapped into)
- Lighter + smaller volume
Role of the cladding
- Protects the core from scratches/moisture
- Increases the critical angle - only rays with large angles to the normal are transmitted (reduces multimode dispersion)
Multimode / multipath dispersion
When light travels down an optical fibre in different paths, rays reach the end at different times and broaden the pulse of light
Problems with multipath dispersion
Pulses of light merge into each other and change the information being transferred
Solutions to multimode dispersion
- Leave bigger gaps between pulses
- Use cladding (and make the initial signal brighter to make up for greater loss of signal)
Material dispersion
- Light of different wavelengths travel at different speeds in glass
- Rays will reach the detector at different times
Solutions to material dispertion
Use monochromatic light
Absorption in optical fibres
Impurities in the glass absorb light
Solutions to absorpsion
- Boost signal at regular intervals
- Use infrared light to minimise absorption
