5.3- TOTAL INTERNAL REFLECTION

Question 1

When a light ray travels from glass into air, where does it refract towards?

Answer 1

refracts away from the normal

Question 2

What happens if the angle of incidence is increased to a certain value known as the critical angle?

Answer 2

light ray refracts along the boundary

Question 3

What does the light ray undergo if the angle of incidence is increased further than the critical angle?

Answer 3

total internal reflection at the boundary, same as if boundary were replace by plane mirror

Question 4

In general, when does total internal reflection only happen if…? (2)

Answer 4

incident substance has a larger refractive index than the other substance

angle of incidence exceeds the critical angle

Question 5

How is the angle of refraction 90° at the critical angle?

Answer 5

as the light ray emerges along the boundary

Question 6

What equation can be given as sin90 = 1?

Answer 6

sinθ(small c) = n2 / n1

n1- refractive index of incident substance
n2- refractive index of other substance

Question 7

What happens to light when in enters a diamond?

Answer 7

it’s split into the colours of the spectrum

Question 8

What is the refractive index of diamond like?

Answer 8

very high refractive index of 2.417

Question 9

What can diamond do as it has a very high refractive index?

Answer 9

separates colours more than any other substance does

Question 10

What does the high refractive index of diamond give it?

Answer 10

critical angle of 24.4°

Question 11

What does diamond having a critical angle of 24.4° mean and how does it cause it to look?

Answer 11

light ray in diamond may be totally internally reflected many times before it emerges, which means its colours spread out more and more
so diamond sparkles with different colours

Question 12

Example of where optical fibres are used in?

Answer 12

medical endoscope

Question 13

What are optical fibres used in medical endoscopes for?

Answer 13

to see inside the body

Question 14

What else is optical fibres used for other than endoscopes?

Answer 14

in communications to carry light signals

Question 15

What happens to a light ray when it reaches a fibre boundary in an optical fibre?

Answer 15

light ray totally internally reflected each time it reaches fibre boundary, even where fibre bends, unless radius of bend too small

Question 16

What is the angle of incidence like at each point where the light ray reaches the boundary? (optical fibres)

Answer 16

at each point where light ray reaches boundary, angle of incidence exceeds critical angle of fibre

Question 17

What does a communication optical fibre allow a pulse of light to do?

Answer 17

allows pulse of light that enters at one end, form transmitter, to reach receiver at other end

Question 18

What do communication optical fibres need to be like?

Answer 18

highly transparent

Question 19

Why do communication optical fibres need to be highly transparent?

Answer 19

to minimise absorption of light

Question 20

What would happen if communication optical fibres were not highly transparent, and so didn’t minimise absorption of light?

Answer 20

amplitude of pulses reduces progressively further they travel in fibre

Question 21

What does each fibre consist of? (optical fibres)

Answer 21

core surrounded by layer of cladding of lower refractive index to reduce light loss from core

Question 22

What would light loss do? (optical fibres)

Answer 22

would reduce amplitude of pulses

Question 23

Where does the total internal reflection take place? (optical fibres)

Answer 23

core-cladding boundary

Question 24

What happens at any point where two fibres are in direct contact if there were no cladding? (optical fibres)

Answer 24

light would cross from one fibre to the other

Question 25

What would it mean if light was to cross from one fibre to another due to there being no cladding? (optical fibres)

Answer 25

signals would not be secure, as they would reach the wrong destination

Question 26

Why must the core be very narrow? (optical fibres)

Answer 26

to prevent modal dispersion

Question 27

Where does modal dispersion occur? (optical fibres)

Answer 27

in a wide core

Question 28

Why does modal dispersion occur in a wide core? (optical fibres)

Answer 28

as light travelling along the axis of the core travels a shorter distance per metre of fibre than light that repeatedly undergoes total internal reflection

Question 29

What would happen to a pulse of light that’s sent along a wide core? (optical fibres)

Answer 29

would become longer than it ought to be

Question 30

What would happen if a pulse of light sent along a wide core becomes too long than it ought to be? (optical fibres)

Answer 30

it would merge with the next pulse

Question 31

When else does pulse dispersion happen?

Answer 31

if white light used instead of monochromatic light (light of single wavelength)

Question 32

Why does the material dispersion happen when white light is used instead of monochromatic light?

Answer 32

speed of light in glass of optical fibre depends on wavelength of light travelling through it

Question 33

How does violet light travel in comparison to red light in glass?

Answer 33

violet light travels more slowly than red light in glass

Question 34

As violet light travels more slowly than red light in glass, what would happen when white light is used instead of monochromatic light? (optical fibres)

Answer 34

difference in speed would cause white light pulses in optical to become stronger, as violet component falls behind faster red component of each pulse

Question 35

So why must the light used be monochromatic? (optical fibres)

Answer 35

prevent pulse merging

Question 36

How many bundles of fibres does a medical endoscope contain?

Answer 36

two bundles of fibres

Question 37

Where is the medical endoscope inserted?

Answer 37

into a body cavity

Question 38

How is the medical endoscope illuminated once inserted into a body cavity?

Answer 38

using light sent through one of the fibre bundles

Question 39

What does the light that forms the image do the (medical endoscope)?

Answer 39

light that forms image travels along fibres to other end of fibre bundle where image can be observed

Question 40

What does the fibre bundle that forms the image have to be like? (medical endoscope)

Answer 40

needs to be coherent bundle

Question 41

What does it means coherent bundle?

Answer 41

fibre ends at each end are in same relative positions