Refractive Index and Critical angles (TIR) Flashcards
What is the formula for the absolute refractive index?
What does each letter stand for?
n = C/Cs
n = refractive index of the material
C - Speed of light in vacuum
Cs - Speed of light in the material
What is Snell’s law?
What does each letter mean?
n1 Sinθ1 = n2 Sinθ2
θ1 - angle of incidence ray in material 1
n1 - refractive index of material 1
θ2 - angle of refraction of ray in material 2
n2 - refractive index of material 2
What is the critical angle?
The incident angle at which the refracted angle is at 90 degrees to the normal
What is the equation for the critical angle?
What’s the condition?
sinθc = n2/n1
Where n1>n2
What is the formula for the relative refractive index between two materials?
1n2 = n2/n1
1n2 - relative refractive index of a boundary (going from one material to another)
n1 - refractive index of material 1
n2 - refractive index of material 2
What happens to a wave when it is refracted?
It’s speed and wavelength change but the frequency stays the same
What are the conditions for a light to refract perpendicular to the normal?
- Angle of incidence is equal to the critical angle
- Light goes from more optically dense to less optically dense
What are the conditions for total internal reflection?
Can only happen if sinθc < 1, or 1n2 < 1
How does total internal reflection happen?
Angles greater than the critical angle
What are optical fibres?
A thin glass or plastic tube. Step index optical fibres have a high refractive index (optically dense) core surrounded by cladding with a lower refractive index (less optically dense) to allow for TIR
Why are optical fibres thin?
So the light always hits the boundary between the fibre and cladding at an angle greater than the critical angle
What are optical fibres used for?
Transmit phone and TV signals
What are the advantages of optical fibres over copper cables?
- Signal can carry more information because light has a higher frequency
- The light doesn’t heat up the fibre (so less energy loss)
- There is no electrical interference
- Cheaper to make
- Minimal signal loss (some does occur)
What is it called when information is lost in optical fibres?
Signal degradation
What is absorbtion in optical fibres?
- Some of the signal’s energy is absorbed by the material the fibre is made from
- Results in a lower amplitude
What are the two types of dispersion and what is their effect?
- Modal and material dispersion
- Results in pulse broadening
What is pulse broadening?
The received signal is broader than the initial signal, broadened pulses can overlap each other, resulting in information loss
What is modal dispersion?
- Light rays entering the optical fibre at different angles
- So different rays take different paths down the fibre
- Rays taking a path straight down the middle of the fibre arrive quicker than rays taking a longer reflected path
What is material dispersion?
- Caused by different wavelengths diffracting by different amounts
- Different wavelengths slow down by different amounts in a material
- Because white light consists of different wavelengths, it takes some parts of the signal to take a longer time to travel
What can reduce material dispersion?
Using monochromatic light
What can reduce modal dispersion?
Use a single mode fibre (only allows light to follow a very narrow path)
What can reduce both material and modal dispersion?
Optical fibre repeaters
- Regenerate the singal every so often
What is a light ray that travels straight down the middle of a fibre called?
Axial ray
What is a light ray that bounces off the sides of a fibre optic called?
Non-axial ray
What happens to light when it goes from a less optically dense medium to a more optically dense medium?
Slows down, gets closer to the normal
What happens to light when it goes from a more optically dense medium to a less optically dense medium?
Speeds up, gets further away from the normal
