Waves 2

Question 1

Define refraction

Answer 1

when a wave changes direction and speed as it enters a medium with a different optical density

Question 2

When a wave is refracted, what else also happens?

Answer 2

the ray is partially refected

Question 3

What do wavefronts join?

Answer 3

wavefronts join points of equal phase

Question 4

What is the relationship between rays and wavefronts?

Answer 4

rays are perpendicular to wavefronts

Question 5

What is Snell’s law?

Answer 5

n1(sin(i)) = n2(sin(r))

Question 6

Define refractive index

Answer 6

the ratio of the speed of light in a vacuum to its speed in a given medium

Question 7

What is the range for the refractive index always?

Answer 7

refractive index is always greater than or equal to 1

Question 8

What refracts more, red light or blue light? Why?

Answer 8

• blue light
• the shorter the wavelength, the greater the angle of refraction
• blue light travels slower in glass than red light, so bends more

Question 9

What are the conditions for total internal reflection to occur?

Answer 9

1) the angle of incidence must be greater than the critical angle
2) the ray must be travelling in the optically denser medium

Question 10

Define the critical angle

Answer 10

the angle of incidence where the angle of refraction equal 90°

Question 11

What is the refractive index of air?

Answer 11

≈1

Question 12

What is an optical fibre?

Answer 12

a narrow tube of glass that can carry light signals using total internal reflection

Question 13

What are the three purposes of the cladding?

Answer 13

1) protects the core from scratching
2) stops two cores coming into contact and the light travelling between them (prevents data corruption and theft)
3) increases the critical angle to reduce the number of possible pathways for the light

Question 14

What is pulse broadening?

Answer 14

when signal in an optical fibre gets wider and it transmits, due to dispersion

Question 15

Why is pulse broadening problematic?

Answer 15

can cause adjacent bits to overlap and become indistinguishable

Question 16

What are the two main reasons for pulse broadening?

Answer 16

1) modal dispersion

2) material dispersion

Question 17

What is modal dispersion?

Answer 17

• where light follows multiple paths of differing lengths

- light following the longer paths arrives after light that had taken a more direct route

Question 18

What is a solution to modal dispersion

Answer 18

• use a fibre with a narrow core

the cladding also helps as a large critical angle limits the number of paths that propagate

Question 19

What is material dispersion?

Answer 19

• dispersion that happens because blue light travels slower in glass compared to red light
• the longer wavelengths in a signal therefore arrive before the shorter wavelengths, spreading the signal out

Question 20

What is the solution to material dispersion?

Answer 20

use monochromatic (single wavelength) light

Question 21

Give three advantages of optical fibres compared to copper cable

Answer 21

• able to carry more simultaneous signals
• faster data transmission
• more secure
• less susceptible to noise
• needs boosting less frequently

Question 22

Give two disadvantages of optical fibres compared to copper cable

Answer 22

• more difficult to modify
• more expensive (to manufacture and install)
• more complicated electronics required for interface and operation

Question 23

What is the wavelength of red light?

Answer 23

around 700nm

Question 24

What is the wavelength of blue light?

Answer 24

around 450nm

Question 25

Define diffraction

Answer 25

the spreading out of a wavefront after passing through a gap or around objects

Question 26

When is diffraction maximised?

Answer 26

when the gap width/obstacle is equal to the wavelength

Question 27

Describe the single slit diffraction pattern?

Answer 27

• wide, bright, central maximum
• subsequent maxima are half the width and progressively dimmer
• fringes appear perpendicular to the slit
• bright and dark regions (maxima and minima) alternate

Question 28

What happens to the single slit diffraction pattern if the slit becomes narrower?

Answer 28

there is a wider, dimmer central maximum

Question 29

What is the difference in the single slit diffraction pattern of blue light compared to red light? Why?

Answer 29

• blue light has a smaller wavelength, therefore less diffraction occurs
• so a narrower, brighter central maximum

Question 30

What is meant by w (fringe width)?

Answer 30

the bright and the dark region

Question 31

How can you reduce the percentage error when measuring w?

Answer 31

measure as many as possible and then divide

Question 32

What is meant for two sources to be coherent?

Answer 32

maintain a constant phase difference

Question 33

What is meant by monochromatic light?

Answer 33

light all of the same wavelength

Question 34

State the diffraction pattern from Young’s double slit experiment

Answer 34

equally spaced dark and bright fringes of equal intensity

Question 35

State the diffraction pattern from Young’s double slit experiment when using white light

Answer 35

• a white central fringe with red edges
• the other maxima would be spectrum
• with blue closest to the centre and red furthest away 𝑤 = (𝜆D)/𝑠

Question 36

Explain the pattern from Young’s double slit experiment

Answer 36

• bright fringes are formed when light constructively interferes
• dark fringes are formed when light destructively interferes

Question 37

What is a diffraction grating?

Answer 37

a glass slide which contains lots of equally spaced vertical slits very close together

Question 38

What does d equal in the equation

d sin θ = nλ?

Answer 38

d equals slit spacing

Question 39

How can you find d from the number of slit per metre?

Answer 39

d = 1 / number of slits per m

Question 40

What is the diffraction pattern of white light though a diffraction grating?

Answer 40

• n = 0 is a white light with red edges
• n = 1 is a spectrum from blue (closest to the centre) to red
• so on for n = 2, n=3, …

Question 41

What is the pattern from a diffraction grating?

Answer 41

sharp equally spaced bright fringes of equal intensity