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 slows down 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 (harder to intercept signals) 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**

Question 41

What is oscillating in the case of a mechanical wave?

Answer 41

Particles are oscillating about their equilibrium position

Question 42

What is oscillating in the case of an electromagnetic wave?

Answer 42

Electric and magnetic fields No particles are involved in oscillations This is why electromagnetic waves can travel through a vacuum

Question 43

What do waves not transfer?

Answer 43

They DO NOT transfer matter

Question 44

Are all longitudinal waves mechanical?

Answer 44

Yes EM waves transfer energy perpendicular to oscillations and so cannot be longitudinal Therefore, any longtudinal wave can only be mechanical

Question 45

Are all mechanical waves longitudinal?

Answer 45

No Mechanical waves can be longitudinal (sound waves) but can also be transverse (string)

Question 46

What occurs when the gradient of a displacement-distance graph for a longitudinal wave is: * Positive * Negative

Answer 46

* When the gradient is positive, rarefactions occur * When the gradient is negative, compressions occur

Question 47

What is partial polarisation and when does it occur?

Answer 47

Where more light is transmitted in one plane than in other planes This occurs during reflection, such as reflection off water, glass or metal

Question 48

How is light intensity emitted related to the angle of the second filter?

Answer 48

Intensity follows a cosine graph where initial intensity is max If initial intensity is 0 it will follow a sine graph

Question 49

How does refractive index affect the speed of light?

Answer 49

Inversely Higher refractive index → Slower speed of light Lower refractive index → Higher speed of light

Question 50

Describe the process of reaching the critical angle

Answer 50

* A ray of light passing from higher to a lower refractive index will refract away from the normal when the angle of incidence \< critical angle and there will be a partially reflected ray * As the angle of incidence increases, the angle of refraction increases * The intensity of the reflected ray increases as the intensity of the refracted ray decreases * When the angle of incidence = critical angle, the angle of refraction is 90 degrees and travels along the boundary. There is still a reflected ray * If the angle of incidence increases further, (angle of incidence \> critical angle), total internal reflection occurs. There is no refracted ray.

Question 51

What is attenuation and how is it overcome?

Answer 51

The decrease in the intensity of a wave over long distances Solutions: * Using very high purity glass * This keeps the intensity as high as possible along the fibre

Question 52

Which end of the visible light spectrum has the highest refractive index?

Answer 52

Violet light (shortest wavelength) (slows down the most in refraction)

Question 53

Which end of the visible light spectrum has the highest refractive index?

Answer 53

Violet light (shortest wavelength) (slows down the most in refraction)