Refraction

Question 1

When does refraction occur?

Answer 1

• when light passes a boundary between two different transparent media
• At the boundary, the rays of light undergo a change in direction

Question 2

Why does a ray of light move towards/ away from a normal

Answer 2

due to the change in speed when travelling in different substances

When light passes into a denser substance the rays will slow down

Question 3

What wave properties change during refraction

Answer 3

speed and wavelength – the frequency of waves does not change

Question 4

What do the components of the refractive index equation mean

Answer 4

Question 5

What is a material with a high refractive index called

Answer 5

optically dense

Question 6

Why is refractive index (n) always bigger than 1

Answer 6

Because the speed of light in a substance will always be less than the speed of light in a vacuum

Question 7

What do the components of snell’s law equation mean

Answer 7

Question 8

When does total internal reflection occur and how does this relate to the angle of incidence

Answer 8

• When the angle of refraction is exactly 90° and the light is refracted along the boundary
• At this point, the angle of incidence is known as the critical angle θc

Question 9

What do the components of the critical angle equation mean

Answer 9

Question 10

Definition for when total internal reflection occurs

Answer 10

The angle of incidence is greater than the critical angle and the incident refractive index n1 is greater than the refractive index of the material at the boundary n2

Question 11

What are the two conditions required for total internal reflection to occur

Answer 11

The angle of incidence > the critical angle
The refractive index n1 is greater than the refractive index n2

Question 12

What are fibre optics (which use total internal reflection) good for

Answer 12

sending high speed light signals over large distances

Question 13

Examples of fibre optics uses

Answer 13

• Communications, such as telephone and internet transmission
• Medical imaging, such as endoscopes

Question 14

What are the three components that make up a fibre optic

Answer 14

• An optically dense core, such as plastic or glass
• A lower optical density cladding surrounding the core
• An outer sheath

Question 15

What is the purpose of the fibre optic outer sheath?

Answer 15

• Prevents physical damage to the fibre
• Strengthens the fibre
• Protects the fibre from the outside from scratches

Question 16

What is the purpose of the fibre optic cladding?

Answer 16

• It protects the core from damage
• It prevents signal degradation through light escaping the core
• It prevents scratching of the core
• It keeps the core away from adjacent fibre cores hence preventing crossover of information to other fibres
• It provides the fibre with strength and prevents breakage given that the core needs to be very thin

Question 17

When does material dispersion occur and why

Answer 17

when white light is used instead of monochromatic light

because different wavelengths of light travel at different speeds:
- Blue light travels slower than red light due to the greater refractive index
- Therefore, the red light reaches the end before the blue light

Question 18

When does modal dispersion occur and why

Answer 18

when the light pulses in the optical fibre spread out due to the different angles of incidence in the original pulse

This is more prominent in wider cores as the light travelling along the axis of the core travels a shorter distance than light undergoing total internal reflection at the core-cladding boundaries
To prevent modal dispersion, the core needs to be very narrow

Question 19

What are the advantages of using a narrower optical fibre core

Answer 19

• Less light is lost by refraction out of the core
• Less overlapping pulses hence reduction of modal dispersion
• The quality of the signal will be better and less distorted
• The signal will be transferred quicker leading to improved data and information transfer

Question 20

When does pulse absorption occur AND what are its effects

Answer 20

• Part of the signal’s energy is absorbed by the fibre
• The signal is attenuated by the core

This reduces the amplitude of the signal, which can lead to a loss of information

Question 21

When does pulse broadening occur AND what are its effects

Answer 21

• is caused by modal and material dispersion
• The consequence of pulse broadening is that different pulses could merge, resulting in a completely distorted final pulse

Question 22

How can pulse broadening and absorption be reduced

Answer 22

• To reduce absorption:
  Use a core which is extremely transparent
  Use of optical fibre repeaters so that the pulse is regenerated before significant absorption has taken place
• To reduce pulse broadening:
  Make the core as narrow as possible to reduce the possible differences in path length of the signal
  Use of a monochromatic source so the speed of the pulse is constant
  Use of optical fibre repeaters so that the pulse is regenerated before significant pulse broadening has taken place

Question 23

How are the wavelength, frequency and speed of light affected when a ray of light travels from air to glass

Answer 23

• wavelength decreases
• frequency stays the same
• speed decreases