3.5 Refraction

Question 1

When does refraction occur?

Answer 1

when light passes a boundary between 2 different transparent media
-the rays if light undergo a change in direction

Question 2

what is the normal?

Answer 2

• the direction of the refraction is taken as the angle from a hypothetical line
• this line is perpendicular to the surface of the boundaries and is represented by a straight dotted line

Question 3

What is the change in direction of refraction dependent on?

Answer 3

which media the light rays pass between

Question 4

What direction does light refract when it passes from less dense to a more dense material?

Answer 4

light bends towards the normal
- e.g. air to glass

Question 5

What direction does light refract when it passes from a more dense to a less dense media?

Answer 5

light bends away from the normal
-e.g. glass to air

Question 6

What direction does light refract when it passes along the normal

Answer 6

the light does not bend at all

Question 7

Why does the direction of light change when refraction occurs?

Answer 7

• due to the change in speed when travelling in different substances
• when light passes into a denser substance the rays will slow down, hence bend towards the normal

Question 8

What properties change during refraction?

Answer 8

speed and wavelength

Question 9

What is the refractive index?

Answer 9

a property of a material which measures how much light slows down when passing through it

Question 10

What is the refractive index used for?

Answer 10

to determine the speed at which light travels within different substances

Question 11

What property does an optically dense material have?

Answer 11

high refractive index
- causes light to travel slower

Question 12

What is critical angle?

Answer 12

• when the angle of refraction is exactly 90˚ the light is reflected across the boundry
• the angle of incidence here is known as the critical angle

Question 13

When does total internal reflection occur?

Answer 13

• when the angle of incidence is greater than the critical angle
• the incident refractive index is greater than the refractive index of the material at the boundary

Question 14

What are the 2 conditions for TIR to occur?

Answer 14

• angle of incidence > the critical angle
• the refractive index of material 1 is greater than the refractive index of material 2

Question 15

What uses to fibre optics have?

Answer 15

• communications, such as telephone and internet transmission
• medical imaging, such as endoscopes

Question 16

What are the 3 main components that make up optical fibres?

Answer 16

• optically dense core, such as glass or plastic
• a lower optical density cladding surrounding the core
• an outersheeth

Question 17

How does TIR occur in an optical fibre?

Answer 17

since the refractive index of the core is > the refraction index of the cladding TIR can occur

Question 18

What is the outer sheath of an optical fibre?

Answer 18

• prevents physical damage to the fibre
• strengthens the fibre
• protects the fibre from the outside from scratches

Question 19

What is the role of cladding in the fibre optics?

Answer 19

• protects the core from damage
• prevents signal degradation through light escaping the core, causing information from signal to be lost
• maintains the quality of the signal whilst keeping it secure
• 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 20

When does material dispersion occur?

Answer 20

when white light is used instead off monochromatic light

Question 21

why does material dispersion occur?

Answer 21

• different wavelengths of light travel at different speeds

Question 22

When does modal dispersion occur?

Answer 22

• 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 TIR at the core cladding boundaries

Question 23

How can modal dispersion be prevented?

Answer 23

• the core needs to be very narrow

Question 24

What are the advantages of using a narrow core?

Answer 24

• less light is lost by refraction out of the core
• there is a smaller change in angle between each reflection, so the angle of incidence is less likely to fall below the critical angle
• 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 25

When does absorption occur?

Answer 25

- when part of the signals energy is absorbed by the fibre - the signal is attenuated by the core

Question 26

What is the effect of absorption?

Answer 26

- reduction of the amplitude of the signal, which can lead to a loss of information

Question 27

What is pulse broadening caused by?

Answer 27

modal and material dispersion

Question 28

What are the consequences of pulse broadening?

Answer 28

different pulses could merge, resulting in a completely distorted final pulse

Question 29

How can you reduce absorption?

Answer 29

- use a core which is extremely transparents - use of optional fibre repeaters so that the pulse is regenerated before significant absorption has taken place

Question 30

How can you reduce pulse broadening?

Answer 30

- make the core as narrow as possible to reduce the possible differences in path length of the signal - use of monochromatic source so the speed of the pulse is constant - use of optical fibre repeaters so the pulse is regenerated before significant pulse broadening has taken place - use of single mode fibre to reduce multi path modal dispersion