Topic 4: Waves Flashcards

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1
Q

Define displacement (x).

A

The distance the medium has moved from the equilibrium position in a specific direction (m).

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2
Q

Define amplitude (x₀).

A

The maximum displacement from the equilibrium position (m).

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3
Q

Define frequency (f).

A

The number of oscillations per second (Hz).

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4
Q

Define period (T).

A

The time for one oscillation (s).

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5
Q

Define wavelength (λ).

A

The shortest distance between two points that are in phase along a wave (m). Eg. crest to crest.

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6
Q

Define wavespeed (v).

A

The distance travelled per unit time by the energy of the wave (or by a wavefront) (ms⁻¹).

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7
Q

Define phase (φ).

A

A measure of how “in step” different particles are.

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8
Q

When does simple harmonic motion (SHM) occur?

A

SHM is a type of oscillation that takes place when the acceleration of (and the force on) an object is;- proportional to its displacement from the equilibrium position.- in the opposite direction to the displacement (i.e. towards the equilibrium position).*The motion is due to a restoring force.

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9
Q

If a graph of acceleration against displacement is plotted what would a straight line and negative gradient show?

A
  • A straight line shows that acceleration and displacement are proportional.- A negative gradient shows that acceleration and displacement are in opposite directions.
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10
Q

Define wave.

A

A movement of energy through a medium.

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11
Q

Describe the two types of wave motion.

A

Longitudinal waves - the particles of the medium vibrate parallel to the direction of the energy transfer. Eg. sound, earthquake P waves. Rarefaction and compression.Transverse waves - the particles of the medium vibrate at right angles to the direction of the energy transfer. Eg. light, earthquake S waves. Crest and trough.

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12
Q

Define intensity.

A

The power per unit area received by an observer. Measured in Wm⁻².

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13
Q

What is the intensity of a wave related to?

A

It is proportional to the square of the amplitude of the wave. I∝A²It is inversely proportional to the square of the distance from the source. I∝x⁻²

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14
Q

Explain the two types of waves.

A

Mechanical waves. These require a medium to travel through. Eg. sound and earthquake waves.Electromagnetic waves. These do not require a medium to travel through i.e. they can pass through a vacuum. All electromagnetic waves are transverse and travel at the speed of light in a vacuum.

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15
Q

Give the order of magnitude for the electromagnetic spectrum.

A

(Roman Men Invented Very Useful Xray Guns)In order of decreasing wavelength: Radiowaves 10²Microwaves 10⁻³Infrared 10⁻⁵Visible 10⁻⁷Ultraviolet 10⁻⁹Xray 10⁻¹²Gamma rays 10⁻¹⁴

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16
Q

Define the refractive index (n).

A

A measure of how fast a wave travels in the medium. The higher the value of n, the slower the wave travels.

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17
Q

Give Snell’s Law.

A

n = sini/sinr*Only applies if the first medium is air. If the first medium is not air, the law given in the data booklet applies.

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18
Q

When does total internal reflection occur?

A

When light is travelling from a more to a less optically dense medium and the angle of incidence at the boundary is greater than the critical angle.

19
Q

Give the formula to work out the critical angle or refractive index.

A

sinθc = 1/n₁

20
Q

Define wavefront.

A

A line connecting points on a wave with the same phase/displacement.

21
Q

How is a ray drawn on a wavefront?

A

A ray is a line drawn to represent the direction a wave is travelling when viewed from above. It is drawn at 90° to the wavefronts.

22
Q

Define diffraction.

A

When waves move past an object or through a gap the waves tend to spread out. This is called diffraction.

23
Q

What is affected in refraction?

A

Speed and wavelength are affected.Frequency stays the same.

24
Q

What is affected in diffraction?

A

Amplitude decreases as the energy is spread out over a longer wavefront.Frequency, speed and wavelength stay the same.

25
Q

Describe constructive interference.

A

When waves meet in phase, they will form a resultant wave with an amplitude equal to the sum of the two individual waves.

26
Q

Describe destructive interference.

A

When waves meet out of phase, they will cancel each other out to give a wave of zero amplitude (assuming both waves are of equal amplitude).

27
Q

Define the principle of superposition.

A

If two or more waves meet, the resultant displacement at any point is found by adding the displacements produced by each individual wave.

28
Q

What are antinodal and nodal lines?

A

Antinodal lines are drawn when waves meet in phase (constructive interference). Path difference = nλ. Antinodes are the points at max displacement.Nodal lines are drawn when waves meet out of phase (destructive interference). Path difference = (n + 1/2)λ Nodes are the points at 0 displacement.

29
Q

What are the units the formula s = λD/d?

A

Metres.s = distance between the bright fringesλ = wavelength of the lightD = distance from the double slits to the interference pattern on a screend = distance between the double slits

30
Q

How do standing waves form?

A

When two waves of the same type meet. They must be;- of the same amplitude- of the same frequency- travelling in opposite directions*Note: Energy is not transferred in standing waves.Eg. when a wave reflects back on itself from a boundary. The inverted reflected wave interferes with the forward moving wave to create a standing wave.

31
Q

What is polarisation?

A

If a wave is polarised, then all the oscillation of the medium is only in one plane. When light is polarised, the electric field vector is oscillating in one plane only.*Note: only transverse waves can be polarised.

32
Q

How can polarised light be produced from unpolarised light?

A
  • Reflection of a surface (Brewster’s Law)- Using polarising filters (Malus’ Law)
33
Q

Discuss Brewster’s Law.

A

When light reflects off a surface, it will be polarised in the plane of the surface. n = tanθ

34
Q

When is polarisation 100% when light reflects of a surface?

A

If the angle between the reflected and refracted ray is 90°.

35
Q

Discuss Malus’ Law.

A

Light is passed through a polarising filter and then an analyser.

36
Q

What happens when unpolarised light is passed through a polariser?

A

The light intensity is reduced by half.

37
Q

nm -> m

A

divide by 10^9

38
Q

Define an optically active substance.

A

One that rotates the plane of polarisation of light that passes through it.

39
Q

What is the graph for SHM?

A

Upside down parabola.

40
Q

What is the graph for intensity against an angle?

A

Parabola with a minimum at 90° and a maximum at 180°.

41
Q

How do sunglasses work?

A

Sunglasses work by blocking the horizontally polarised light in order to stop light glare from reflected rays off of water. So they have vertical slits only.

42
Q

Describe interference with polarised light.

A

Two waves must have the same plane of polarisation and phase in order to produce an interference pattern. If waves of two different polarised light meet, a wave of a different plane of polarisation is produced.

43
Q

What type of wave are standing waves in pipes?

A

Sound waves which are longitudinal so the particles are vibrating horizontally.

44
Q

Do waves travel faster in deep water or shallow water?

A

Deep water.