Refractive Index and Critical angles (TIR)

Question 1

What is the formula for the absolute refractive index?
What does each letter stand for?

Answer 1

n = C/Cs

n = refractive index of the material
C - Speed of light in vacuum
Cs - Speed of light in the material

Question 2

What is Snell’s law?
What does each letter mean?

Answer 2

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

Question 3

What is the critical angle?

Answer 3

The incident angle at which the refracted angle is at 90 degrees to the normal

Question 4

What is the equation for the critical angle?
What’s the condition?

Answer 4

sinθc = n2/n1

Where n1>n2

Question 5

What is the formula for the relative refractive index between two materials?

Answer 5

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

Question 6

What happens to a wave when it is refracted?

Answer 6

It’s speed and wavelength change but the frequency stays the same

Question 7

What are the conditions for a light to refract perpendicular to the normal?

Answer 7

• Angle of incidence is equal to the critical angle
• Light goes from more optically dense to less optically dense

Question 8

What are the conditions for total internal reflection?

Answer 8

Can only happen if sinθc < 1, or 1n2 < 1

Question 9

How does total internal reflection happen?

Answer 9

Angles greater than the critical angle

Question 10

What are optical fibres?

Answer 10

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

Question 11

Why are optical fibres thin?

Answer 11

So the light always hits the boundary between the fibre and cladding at an angle greater than the critical angle

Question 12

What are optical fibres used for?

Answer 12

Transmit phone and TV signals

Question 13

What are the advantages of optical fibres over copper cables?

Answer 13

• 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)

Question 14

What is it called when information is lost in optical fibres?

Answer 14

Signal degradation

Question 15

What is absorbtion in optical fibres?

Answer 15

• Some of the signal’s energy is absorbed by the material the fibre is made from
• Results in a lower amplitude

Question 16

What are the two types of dispersion and what is their effect?

Answer 16

• Modal and material dispersion
• Results in pulse broadening

Question 17

What is pulse broadening?

Answer 17

The received signal is broader than the initial signal, broadened pulses can overlap each other, resulting in information loss

Question 18

What is modal dispersion?

Answer 18

• 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

Question 19

What is material dispersion?

Answer 19

• 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

Question 20

What can reduce material dispersion?

Answer 20

Using monochromatic light

Question 21

What can reduce modal dispersion?

Answer 21

Use a single mode fibre (only allows light to follow a very narrow path)

Question 22

What can reduce both material and modal dispersion?

Answer 22

Optical fibre repeaters
- Regenerate the singal every so often

Question 23

What is a light ray that travels straight down the middle of a fibre called?

Answer 23

Axial ray

Question 24

What is a light ray that bounces off the sides of a fibre optic called?

Answer 24

Non-axial ray

Question 25

What happens to light when it goes from a less optically dense medium to a more optically dense medium?

Answer 25

Slows down, gets closer to the normal

Question 26

What happens to light when it goes from a more optically dense medium to a less optically dense medium?

Answer 26

Speeds up, gets further away from the normal