Waves 1 - Progressive waves and Refraction

Question 1

What is a progressive wave?

Answer 1

Oscillations that have a resultant transfer of energy in one direction

Question 2

How are mechanical and electromagnetic waves different?

Answer 2

Mechanical waves require a medium to oscillate through

Electromagnetic waves don’t require matter (oscillate through electric and magnetic fields)

Question 3

What makes a wave transverse?

Answer 3

Oscillations are perpendicular to the transfer of energy

Question 4

What makes a wave longitudinal?

Answer 4

Oscillations are parallel to the transfer of energy

Question 5

What 2 properties do all electromagnetic waves possess?

Answer 5

1. Always transverse
2. Propagate with velocity of 3×10⁸ms^-1 through vacuum

Question 6

Name 3 longitudinal waves

Answer 6

• Sound
• P-waves (Earthquakes)
• Water waves (beneath surface)

Question 7

Name 3 transverse waves

Answer 7

• E-M waves (Light, X-rays, UV etc)
• Waves on string
• S-waves (Earthquakes)
• Water waves (surface)

Question 8

List in order all waves on the E-M spectrum

Answer 8

Question 9

How are displacement and amplitude of a wave different?

Answer 9

Displacement → Current distance of a point from the equilibrium position

Amplitude → Maximum distance a point reaches from equilibrium position

Question 10

Why do all points on a progressive wave have the same amplitude?

Answer 10

All points have the same maximum displacement from equilibrium position

Question 11

What is the time period of a wave?

Answer 11

Time taken for each particle to complete one full oscillation

(Return to same position)

Question 12

How is frequency of a wave defined?

Answer 12

The number of complete oscillations per second

Question 13

What is the wavelength of a wave?

Answer 13

Distance between two adjacent corresponding points on a wave

(Same displacement, no phase difference)

Question 14

What is the phase difference between A and B on this progressive wave?

Answer 14

360° ∼ 0°

2π∼ 0π

Question 15

What is the phase difference between A and B on this progressive wave?

Answer 15

180°

π ∼ Antiphase

Question 16

What is the phase difference between points A and B on this progressive wave?

Answer 16

540° ∼ 180°

3π ∼ π ∼ antiphase

Question 17

How is phase difference calculated in degrees?

Answer 17

Question 18

How is phase difference calculated in radians?

Answer 18

Question 19

How do you convert from degrees → radians?

Answer 19

Question 20

What is the phase difference between A and B on this progressive wave?

Answer 20

420° ∼ 60°

14π/6 ∼ π/3

Question 21

How are frequency and wavelength related?

Answer 21

Question 22

What are the 2 key features of longitudinal waves?

Answer 22

Compressions and rarefactions

Question 23

Why can’t sound waves be polarised?

Answer 23

Only transverse waves can be polarised

(Sound is longitudinal)

Question 24

What is the final intensity?

Answer 24

1. Light vertically polarised through first grating
2. Vertically p[olarised light can’t pass through horizontal grating
3. Final intensity = 0

Question 25

What is the final intensity?

Answer 25

1. Light vertically polarised through first grating (intensity halves)
2. Vertically polarised light passes through second grating
3. Final intensity = ½

Question 26

How do sunglasses reduce glare?

Answer 26

1. When sunlight reflects off surfaces it is polarised
2. Sunglasses have filter to block polarised light
3. Only unpolarised light passes through

Question 27

What is the refractive index of a material?

Answer 27

Ratio of speed of light in a vacuum : speed light passes through material

(The greater n > the more light slows down)

Question 28

How does θ₂ compare to θ₁?

Answer 28

θ₂ > θ₁

(Light speeds up and bends away from normal)

Question 29

How does θ₂ compare to θ₁?

Answer 29

θ₂ < θ₁

(Light speeds up and bends towards normal)

Question 30

Is the light refracting here?

Answer 30

Yes

It hasn’t bent towards or away from normal

But it has slowed down

Question 31

How does refraction affect the frequency of a wave?

Answer 31

Frequency does not change

(But wavespeed and wavelength do)

Question 32

What is dispersion?

Answer 32

Different wavelength refract by different amounts

So light passing through a prism separates into wavelengths

Question 33

What is wrong here?

Answer 33

In Snell’s law θ₁ is the angle between normal and incident ray

Question 34

What are the 2 conditions for total internal reflection?

Answer 34

1. θ₁ > θ_c
2. n₂ < n₁

Question 35

How is the critical angle calculated?

Answer 35

Question 36

How do you calculate the angle of incidence in the fibre?

Answer 36

Using basic geometry (angles in triangle add to 180°)

Question 37

Why are optical fibres better than copper cables?

Answer 37

1. Information transmission faster
2. More information can be transmitted
3. Less energy loss (copper heats up)

Question 38

In optical fibres what does cladding do?

Answer 38

1. Protects the core from scratches and spills
2. Stops data loss to adjacent fibres
3. Increases critical angle (reducing modal dispersion)

Question 39

What is modal dispersion and how is it combatted?

Answer 39

Different modes (angles) take different amount of time to propagate through an optical fibre

Leads to pulse broadening

Combatted by making core narrow and using cladding with low n (increasing θ_c)

Question 40

What is spectral (material) dispersion and how is it combatted?

Answer 40

Different wavelengths (colours) of light refracted by different amounts

Leads to pulse broadening

Combatted using monochromatic light