Waves

Question 1

What is the equation for Snell’s Law?

Answer 1

n1/n2 = sinθ2 / sinθ1

Question 2

What is an optical fibre?

Answer 2

A thin glass/plastic fibre that transmits light signals via total internal reflection

Question 3

What is a step-index optical fibre?

Answer 3

An optical fibre with a uniform refractive index in the core and a smaller refractive index in the cladding

Question 4

What is material dispersion?

Answer 4

*When white light is used instead of monochromatic light
* Diff wavelengths (from white) travel at different speeds
* Causing the light to spread out as it transmits
* Pulse broadening

Question 5

What does multimode and monomode mean?

Answer 5

multimode = wide optical fibre core
monomode = narrow optical fibre core

Question 6

What can be done to prevent material dispersion?

Answer 6

Use monochromatic light (single wavelength)

Question 7

What is modal dispersion?

Answer 7

When beams of light enter the fibre at diff angles -> each beam undergoes TIR a diff no. of times -> each beam reaches the end at diff times -> pulse broadening

Question 8

What is pulse broadening?

Answer 8

When the duration of a signal increases due to modal or material dispersion

Question 9

What are the 2 conditions for total internal reflection?

Answer 9

1. angle of incidence > critical angle
2. n1 > n2

Question 10

When the ray goes from a low to a high refractive index, light bends _______ the normal.

Answer 10

towards

Question 11

When the ray goes from a high to a low refractive index, light bends _______ the normal.

Answer 11

away

Question 12

When a pulse of light travels down an optical fibre the amplitude decreases. Why is this?

Answer 12

Energy from the signal is absorbed by the optical fibre

Question 13

Why does light change direction when travelling from one substance to another?

Answer 13

Light has different speeds in different substances:
-when light travels towards a denser substance it slows down -> bends toward the normal

Question 14

The only properties that change during diffraction are _______ and _______. ____________ does not change.

Answer 14

speed / wavelength / frequency

Question 15

Why do the intensity and amplitude slightly decrease upon refraction?

Answer 15

A small portion of light also reflects

Question 16

What is the refractive index (n)?

Answer 16

Shows you how much light slows down when passing through a material

Question 17

What is the critical angle?

Answer 17

The critical angle is the angle of incidence when the angle of refraction is exactly 90° and the light is refracted along the boundary

Question 18

What is the equation to find the critical angle?

Answer 18

sin θc = n2 / n1

Question 19

What are 2 uses of optical fibres

Answer 19

• Communication: Internet transmission and telephone
• Medical: endoscope

Question 20

How do optical fibres allow TIR to occur?

Answer 20

The refractive index of the core is more than the refractive index of the cladding
n cladding < n core

Question 21

Why is cladding required on optical fibres?

Answer 21

• protects core from damage
• prevents signal loss through light escaping the core
• prevents information from transferring between different cores in a bundle

Question 22

What is phase difference?

Answer 22

How much a wave lags behind another wave

Question 23

What is phase difference?

Question 24

Why must the optical fibre core be protected from scratches?

Answer 24

• water can get into scratch -> increase n to become higher than the core n -> prevent TIR -> signal lost

-scratch can alter angle signal where light TIR -> if angle lowered below critical angle -> prevents TIR -> signal lost

Question 25

What is absorption?

Answer 25

where energy is lost as the signal is transferred -> loss of amplitude but doesn’t affect frequency

Question 26

What is absorption?

Answer 26

where energy is lost as the signal is transferred -> loss of amplitude but doesn’t affect frequency

Question 27

What does coherent mean?

Answer 27

The waves must have the same:
-frequency
-wavelength
-have a fixed phase relation

Question 28

What is path difference?

Answer 28

The difference in the length travelled between 2 waves to get to a certain point

Question 29

How do you calculate path difference for 2 coherent wave sources?

Answer 29

(When path difference is a multiple of λ)
path difference = nλ

Question 30

How do you calculate path difference for waves that are exactly out of phase? (destructive)

Answer 30

(When path difference is whole no. + 1/2 λ)
path difference = (n+1/2)λ

Question 31

What did Young do to ensure the 2 waves were coherent in his experiment?

Answer 31

• it is tricky to ensure 2 sources are coherent
• laser produces monochromatic coherent light
• the 2 slits act as 2 identical sources of laser light
• slits have to be the same size to have a comparable wavelength to diffract

Question 32

What must you do to ensure that the fringe measurement is accurate?

Answer 32

Measure across lots of fringes and divide by the no. of fringes to get an average

Question 33

What would we see if we did Young’s double slit experiment with white light?

Answer 33

Instead of bright and dark fringes
-central fringe is white light
-all fringes are more spread out
-side fringes have a spectrum of visible colours
-blue diffracts less than red -> blue is nearer the centre