Refraction

Question 1

What is the absolute refractive index (n)?

Answer 1

Its the ratio between the speed of light in a vacuum and the speed of light in a material

Question 2

What is the refractive index of air?

Answer 2

1

Question 3

What is the speed of light in a vacuum?

Answer 3

3x10^8

Question 4

What is the equation for refractive index?

Answer 4

n=c/cs

Question 5

What happens when optical density increases?

Answer 5

Light slows down more

Question 6

What is optical density measured by?

Answer 6

Refractive index (higher optical density = higher refractive index)

Question 7

What is a relative refractive index a property of?

Answer 7

refractive index is a property between the boundary of two materials (different for every pair)

Question 8

What is a relative refractive index (1n2)?

Answer 8

It is the ratio of speed of light in material 1 to speed of light in material 2

Question 9

What is the equation for relative refractive index?

Answer 9

1n2 = c1/c2. Combine with n=c/cs and gives 1n2 = n2/n1

Question 10

What is Snell’s laws?

Answer 10

n1sin(i) = n2sin(r)

Question 11

What do you do if light passes through a boundary?

Answer 11

Use Law of Refraction to find angles/refractive index

Question 12

What is the angle of incidence?

Answer 12

The angle that incoming light makes with the normal

Question 13

What is the angle of refraction?

Answer 13

The angle the refracted ray makes

Question 14

Why does the speed of light angle change between materials?

Answer 14

because the materials have different optical densities (direction changes)

Question 15

When does light bend away from the normal?

Answer 15

When light goes from a more optically dense material to a less optically dense material

Question 16

When does light bend towards the normal?

Answer 16

When light goes from a less optically dense material to a more optically dense material

Question 17

How do you find the critical angle?

Answer 17

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

Question 18

How do you work out critical angle?

Answer 18

sin(c) = n2/n1 (snells law rearranged)

Question 19

When cant refraction occur?

Answer 19

At angles greater than the critical

Question 20

What happens at angles greater than the critical?

Answer 20

Light is refracted back into the material, this is called total internal reflection

Question 21

Learn and draw diagrams for total internal reflection

Answer 21

Have fun

Question 22

What does an optical fibres do?

Answer 22

They carry light signals over long distances and round corners using total internal reflection

Question 23

What is an optical fibre?

Answer 23

A very thin flexible tube of glass or plastic fibres

Question 24

Describe a step-index optical fibre and draw/label one

Answer 24

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

Question 25

What does the cladding do?

Answer 25

• Allows total internal reflection as it has a lower refractive index than the core
• It protects the fibre from scratches (light doesn’t escape)

Question 26

What are optical fibres used to transmit?

Answer 26

Phone and cable tv signals

Question 27

What are the advantages of optical fibres over electric copper cables?

Answer 27

• 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

Question 28

What happens when light is shone into an optical fibre?

Answer 28

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)

Question 29

What can signal in an optical fibre be degraded by?

Answer 29

Absorption or dispersion

Question 30

What happens in absorption?

Answer 30

• Signal energy is absorbed by material
• This causes amplitude to be reduced
• Learn diagrams

Question 31

What happens in dispersion?

Answer 31

• Signal broadens (pulse broadening) - bigger than initial signal
• Broadened pulses overlap (signal loss)
• Learn diagrams

Question 32

What are the two types of dispersion?

Answer 32

Modal and material

Question 33

What happens in modal dispersion and how do you reduce it?

Answer 33

• 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)

Question 34

What happens in material dispersion and how do you reduce it?

Answer 34

• 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

Question 35

What is another way to reduce signal degradation?

Answer 35

Optical fibre repeaters, can regenerate signal every so often - which reduces absorption and dispersion