Refraction, Diffraction & Interference

Question 1

Define coherence

Answer 1

Coherent waves have a fixed phase difference and the same frequency and wavelength

Question 2

Why is a laser useful in showing interference and diffraction?

Answer 2

It produces monochromatic (same wavelength/colour) light so diffraction and interference patterns are more defined

Question 3

What was Young’s double-slit experiment?

Answer 3

A single light source is directed towards two slits, which each act as a coherent source, producing constructive and destructive interference → interference pattern

Question 4

Describe the interference pattern created using white light

Answer 4

Bright white central maximum, with alternating spectral fringes of decreasing intensity (violet closest, red furthest)

Question 5

Why does an interference pattern form when light is passed through a single slit?

Answer 5

Light diffracts at the slit; in-phase waves interfere constructively (bright fringes), out-of-phase waves interfere destructively (dark fringes)

Question 6

Increasing the slit width increases the width of the central diffraction maximum. True or False?

Answer 6

False – increasing slit width reduces diffraction, central maximum becomes narrower and more intense

Question 7

What is the approximate refractive index of air?

Answer 7

1

Question 8

When light enters a more optically dense medium does it bend towards or away from the normal?

Answer 8

Towards the normal

Question 9

When does total internal reflection occur?

Answer 9

At a boundary to a less dense medium when the angle of incidence > critical angle

Question 10

What is the purpose of the cladding in a step index optical fibre?

Answer 10

Protects core from scratches; allows TIR as it has lower refractive index than core

Question 11

How does signal degradation by absorption in an optical fibre affect the received signal?

Answer 11

Part of the signal energy is absorbed → reduced amplitude

Question 12

What is pulse broadening?

Answer 12

Received signal is wider than original, causing overlap and information loss

Question 13

How does modal dispersion cause pulse broadening?

Answer 13

Rays enter fibre at different angles, travel different paths/times → pulse broadening

Question 14

What is material dispersion?

Answer 14

Different wavelengths travel at different speeds in fibre → arrive at different times → pulse broadening

Question 15

How can modal dispersion be reduced?

Answer 15

Use a single-mode fibre (very narrow core)

Question 16

How can material dispersion be reduced?

Answer 16

Use monochromatic light

Question 17

How can both absorption and dispersion be reduced?

Answer 17

Use optical fibre repeaters to regenerate the signal

Question 18

State the advantages of optical fibres over traditional copper wires

Answer 18

More information capacity (high frequency), no heat loss, no electrical interference, cheaper, very fast

Question 19

What path does a light ray take when the angle of incidence = critical angle?

Answer 19

Travels along the boundary (refraction angle = 90°)

Question 20

What formula can be used to find the critical angle?

Answer 20

sinC = n2/n1 where n1 > n2

Question 21

What is the critical angle of water-air boundary (n_water = 1.33)?

Answer 21

C = sin⁻¹(1/1.33) = 48.8°

Question 22

Using Snell’s law, find r when n1=1.23, n2=1.53, i=32°

Answer 22

r = 25.2°

Question 23

Glass (n=1.5) vs Water (n=1.33), which is more optically dense?

Answer 23

Glass

Question 24

What formula is used to determine refractive index?

Answer 24

n = c/v (c = 3×10^8 m/s in vacuum, v = speed in medium)

Question 25

State 2 applications of diffraction gratings

Answer 25

(1) Splitting starlight into line absorption spectra to identify elements, (2) X-ray crystallography to measure atomic spacing

Question 26

Derive dsinθ = nλ

Answer 26

Path difference = nλ → using geometry sinθ = λ/d → rearrange dsinθ = nλ

Question 27

When light changes from blue to red in diffraction grating, do orders get closer?

Answer 27

No – red has longer λ → more diffraction → orders further apart

Question 28

What is diffraction?

Answer 28

The spreading out of waves when passing through/around a gap

Question 29

How did Young’s double slit experiment prove light is a wave?

Answer 29

Showed diffraction and interference patterns → wave properties

Question 30

What are 4 safety precautions when using a laser?

Answer 30

Wear goggles, avoid reflective surfaces, display warning sign, never shine at person

Question 31

What formula is associated with Young’s double slit experiment?

Answer 31

w = λD/s (w=fringe spacing, D=screen distance, s=slit separation)

Question 32

Laser double-slit: maxima spacing=0.04m, slit sep=0.2mm, D=15m. Find λ

Answer 32

λ = ws/D = (0.04×0.2×10^-3)/15 = 5.3×10^-7 m

Question 33

What is path difference?

Answer 33

Difference in distance travelled by 2 waves

Question 34

How could you investigate stationary sound waves?

Answer 34

Speaker at tube end + powder → shaken from antinodes, settles at nodes. Node spacing = ½λ

Question 35

What is the frequency of 1st harmonic: L=2m, m=0.03kg, mass=2kg?

Answer 35

T=19.62N, μ=0.015kg/m, f = (1/4L)√(T/μ) = 9.0 Hz

Question 36

What is the speed of a wave with f=10GHz, λ=6cm?

Answer 36

c = fλ = (10×10^9)(0.06) = 6×10^8 m/s

Question 37

What is ‘phase’?

Answer 37

Position of a point in the wave cycle (radians, degrees, or fraction of cycle)

Question 38

True or False: Only light can produce interference patterns

Answer 38

False – sound and all EM waves can produce interference patterns