Refraction Flashcards
What is the absolute refractive index (n)?
Its the ratio between the speed of light in a vacuum and the speed of light in a material
What is the refractive index of air?
1
What is the speed of light in a vacuum?
3x10^8
What is the equation for refractive index?
n=c/cs
What happens when optical density increases?
Light slows down more
What is optical density measured by?
Refractive index (higher optical density = higher refractive index)
What is a relative refractive index a property of?
refractive index is a property between the boundary of two materials (different for every pair)
What is a relative refractive index (1n2)?
It is the ratio of speed of light in material 1 to speed of light in material 2
What is the equation for relative refractive index?
1n2 = c1/c2. Combine with n=c/cs and gives 1n2 = n2/n1
What is Snell’s laws?
n1sin(i) = n2sin(r)
What do you do if light passes through a boundary?
Use Law of Refraction to find angles/refractive index
What is the angle of incidence?
The angle that incoming light makes with the normal
What is the angle of refraction?
The angle the refracted ray makes
Why does the speed of light angle change between materials?
because the materials have different optical densities (direction changes)
When does light bend away from the normal?
When light goes from a more optically dense material to a less optically dense material
When does light bend towards the normal?
When light goes from a less optically dense material to a more optically dense material
How do you find the critical angle?
1) Shine light into perspex, gradually moving it round
2) Angle of incidence reaches critical angle when light is refracted at 90 degrees (light along boundary)
3) Happens for any boundary where light goes from more optically dense to less optically dense
How do you work out critical angle?
sin(c) = n2/n1 (snells law rearranged)
When cant refraction occur?
At angles greater than the critical
What happens at angles greater than the critical?
Light is refracted back into the material, this is called total internal reflection
Learn and draw diagrams for total internal reflection
Have fun
What does an optical fibres do?
They carry light signals over long distances and round corners using total internal reflection
What is an optical fibre?
A very thin flexible tube of glass or plastic fibres
Describe a step-index optical fibre and draw/label one
They have a high refractive index (optically dense) core surrounded by cladding with a lower refractive index (less optically dense), this allows total internal reflection
What does the cladding do?
- Allows total internal reflection as it has a lower refractive index than the core
- It protects the fibre from scratches (light doesn’t escape)
What are optical fibres used to transmit?
Phone and cable tv signals
What are the advantages of optical fibres over electric copper cables?
- Signal can carry more info because light has a high frequency
- No electrical interference
- Cheaper to produce
- Signal can travel far, quickly with minimal signal loss
- Light doesnt heat the fibre, so no energy lost as heat
What happens when light is shone into an optical fibre?
It is narrow so light hits the boundary between cladding and fibre at an angle greater than the critical. Light totally internally reflects from one end to the other (shape of the fibre doesnt matter)
What can signal in an optical fibre be degraded by?
Absorption or dispersion
What happens in absorption?
- Signal energy is absorbed by material
- This causes amplitude to be reduced
- Learn diagrams
What happens in dispersion?
- Signal broadens (pulse broadening) - bigger than initial signal
- Broadened pulses overlap (signal loss)
- Learn diagrams
What are the two types of dispersion?
Modal and material
What happens in modal dispersion and how do you reduce it?
- Caused by light entering optical fibre at different angles
- Causes light rays to take different paths (some go straight down middle)
- Can be reduced by using a single-mode fibre (light only allowed to follow narrow path)
What happens in material dispersion and how do you reduce it?
- Caused by different amounts of refraction by different wavelengths
- Different wavelengths of light slow down by different amounts of material (some at different speed as light is made up of different wavelength)
- Monochromatic light can stop material dispersion
What is another way to reduce signal degradation?
Optical fibre repeaters, can regenerate signal every so often - which reduces absorption and dispersion
