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Physics A Level > 4.4 Waves > Flashcards

4.4 Waves Flashcards

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1
Q

What is a transverse wave? (2)

A

Oscillations are perpendicular to the direction of energy transfer
The particles vibrate at 90° to the motion of the wave

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2
Q

What is a longitudinal wave? (2)

A

Oscillations are parallel to the direction of energy transfer
The particles vibrate in the same direction as the wave

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3
Q

What are examples of transverse waves? (4)

A

Water waves
EM waves
Strings
S-waves

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4
Q

What are examples of longitudinal waves? (2)

A

Sound waves

P-waves

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5
Q

What is displacement?

A

The distance of a particle from the equilibrium position (s), metres

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6
Q

What is amplitude?

A

Maximum displacement from equilibrium (A), metres

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7
Q

What is wavelength?

A

Distance between two identical points on adjacent waves (λ), metres

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8
Q

What is period?

A

Time taken for one wavelength to pass a given point (T), seconds

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9
Q

What is frequency?

A

Number of wavelengths passing a given point per unit time (f), Hz

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10
Q

What is wave speed?

A

Distance travelled by wave per unit time (v/c), ms^-1

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11
Q

How can an oscilloscope be used to determine frequency? (4)

A

An oscilloscope plots a graph of PD against time
Horizontal squares (timebase) represent a certain time interval
The height represents PD
From the timebase the period can be determined and used to find the frequency

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12
Q

What is the equation linking frequency and time period?

A

f = 1/T

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13
Q

What is the wave equation?

A

v = fλ

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14
Q

How is the wave equation derived? (2)

A

Since v = d/t, in the time for one cycle a wave travels one wavelength (v = λ/t)
As T = 1/f:
v = λ / (1/f) so v = λf

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15
Q

What is a history graph? (2)

A

Displacement-time

Shows how the position of a single particle series with time as a wave passes through (time period)

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16
Q

What is a snapshot graph? (2)

A

Displacement-distance

Shows entire wave at one moment (wavelength)

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17
Q

What is phase difference?

A

The difference in the relative positions of two or more particles in the same wave or the relative points in the cycle of two or more different waves

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18
Q

What is phase? (2)

A

Describes the current point in the cycle of something that changes periodically, expressed either in radians or degrees
One complete wave = 2π radians

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19
Q

How can the direction a particle is moving be determined?

A

Draw the wave as it would look a short amount of time later in order to see whether specific particles are moving up or down

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20
Q

What is antiphase?

A

A phase difference of 180°

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21
Q

What is refraction? (4)

A

Refraction occurs when a wave passes from one median to another
If the wave slows down it refracts towards the normal and vice versa
There is always partial reflection
Refraction affects wavelength but not frequency

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22
Q

What is reflection? (3)

A

Reflection occurs when a wave changes direction at a boundary between two media
The angle of incidence = the angle of reflection
The wavelength and frequency remains the same

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23
Q

What is diffraction? (3)

A

Diffraction occurs when a wave passes through a gap or around an obstacle
Speed, wavelength and frequency stay the same
Diffraction effects are most significant when the gap is similar in size to the wavelength

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24
Q

What is dispersion?

A 
When white light enters a prism it is refracted, first as it enters the prism and again as it leaves
Longer wavelengths (red) are refracted less than shorter wavelengths (purple)
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25
Q

What is polarisation? (2)

A

The particles oscillate along one direction only, confining the wave to a single plane
The plane of oscillation is both where the particles oscillate and the direction they travel in

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26
Q

Which waves can be polarised?

A

Transverse waves

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27
Q

Why can’t longitudinal waves be plane polarised?

A

The oscillations are already limited to one plane (the direction of energy transfer)

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28
Q

What is partial polarisation?

A

When transverse waves reflect, there are more waves oscillating in a particular plane but the wave is not completely polarised

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29
Q

What is the relationship between intensity and distance? (3)

A

Intensity (Wm^-2) = Power (W) / Area (m^2)
When a wave travels from a point source, they spread across the surface area of a sphere: I = P / 4πr^2
Intensity is proportional to 1/(r^2)

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30
Q

What is the relationship between intensity and amplitude? (3)

A

Intensity is proportional to amplitude^2
Further away from a source, intensity decreases, causing a decreasing amplitude which in turn leads to a reduced average speed.
Halving the amplitude, halves the speed which causes the kinetic energy to be four times as small

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31
Q

What is an electromagnetic wave? (3)

A

A transverse wave
They require no medium to travel through
Electric and magnetic waves oscillating at right angles to each other

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32
Q

Wavelengths - Radio Waves

A

> 10^6 metres to 10^-1 metres

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33
Q

Wavelengths - Microwaves

A

10^-1 metres to 10^-3 metres

34
Q

Wavelengths - Infrared

A

10^-3 metres to 7x10^-7 metres

35
Q

Wavelengths - Visible Light

A

7x10^-7 metres to 4x10^-7 metres

36
Q

Wavelengths - Ultraviolet

A

4x10^-7 metres to 10^-8 metres

37
Q

Wavelengths - X-rays

A

10^-8 metres to 10^-13 metres

38
Q

Wavelengths - Gamma Rays

A

10^-10 metres to <10^-16 metres

39
Q

How are X-rays and gamma rays classified? (2)

A

Not by wavelength due to overlapping ranges

By origin: X-rays are emitted from fast-moving electrons and gamma rays come from unstable atomic nuclei

40
Q

What speed do EM waves travel at?

A

3x10^8 ms^-1 through a vacuum

41
Q

What is a polarising filter?

A

Each filter only allows waves with a particular orientation to pass through

42
Q

What are Polaroid filters? (2)

A

Plastic films that polarise light
Two filters on top of each other will let varying amounts of light through as they are turned through 360° depending on which direction the films are polarising the light

43
Q

How are microwaves polarised?

A

Using a metal grille

44
Q

What is intensity?

A

The energy incident per square metre of a surface per second

45
Q

When is there no refraction?

A

When a wave enters perpendicular to the new medium (along the normal)

46
Q

What is optical density measured using?

A

The refractive index

47
Q

What is the refractive index?

A

The ratio of the velocity of light in a vacuum to its velocity in a specified medium

48
Q

How is absolute refractive index found?

A

n = c / vm

n: refractive index (>1 except for air)
c: speed of light in a vacuum
vm: speed of light in the specified medium

49
Q

How is relative refractive index calculated?

A

n2 / n1 = v1/v2

50
Q

What is Snell’s law? (2)

A

The ratio of the wines of the angles of incidence and refraction is equivalent to the ratio of the velocities in the two media

nsinθ = constant

51
Q

What is the critical angle?

A

The largest angle at which refraction out of a denser medium is just possible

52
Q

What is total internal reflection?

A

When all light hitting the boundary between two media is reflected back into the incident material

53
Q

What are the conditions for total internal reflection to take place? (2)

A

Incident substance has a larger refractive index

Angle of incidence is greater than the critical angle

54
Q

What is the principle of superposition?

A

When two or more waves of the same type exist at the same place, the resultant wave will be found by adding the displacements of each wave

55
Q

How does a steady interference pattern form?

A

Two waves with a constant phase difference and the same frequency interfere over a period of time (coherent sources)

56
Q

What is path difference? (2)

A

The difference between two identical points on interfering waves
Measured by number of wavelengths

57
Q

What is constructive interference? (2)

A 
When crests (or troughs) of coherent waves coincide, creating an amplified wave
The interfering waves are in phase (nλ)
58
Q

What is destructive interference? (2)

A

When the crest of one wave aligns with the trough of another the displacements cancel out
The waves are in anti phase (n+1/2 λ)

59
Q

How is the path difference of a two source interference pattern found?

A

Number of wavelengths from one source to desired point - number of wavelengths from other source to point

60
Q

How is two source interference created with sound waves? (3)

A

Set up speakers a known distance apart and emit sound waves
Walk along a straight line a set distance from the speakers and cover the other ear
Maxima and minima intensities will be heard

61
Q

How is two source interference created with microwaves? (2)

A

Set up metal sheets in front of a transmitter to form a double slit with a gap equal to the wavelength of the microwaves to cause maximum diffraction
Move the receiver along a fixed line in front of the double slit and record the points where maximum and minimum readings are found

62
Q

How is wavelength calculated from a two source interference pattern?

A

λ = ax / D
λ - wavelength (m)
a - gap spacing (between double slit) (m)
x - fringe spacing (maximum intensity to maximum intensity) (m)
D - distance from gap to measuring plane (m)

63
Q

What is the Young double slit experiment using light? (4)

A

A colour filter can be used to make a source of light monochromatic
A single slit makes the light waves in phase
A double slit creates the interference pattern
Fringes are created on a screen, showing that light behaves like a wave

64
Q

What happens when white light is used in the Young double slit experiment?

A

The central white fringe is surrounded by coloured fringes

65
Q

What does single slit diffraction look like?

A

A bright central band, double the width of alternating bands on either side

66
Q

What does double slit diffraction look like?

A

Uniform fringes where light waves interfere (single slit diffraction superimposed on top)

67
Q

What is a stationary wave?

A

When two waves of the same frequency (usually the same amplitude) are travelling in opposite directions in the same medium, they interfere with each other

68
Q

What is a node? (2)

A 
A point on a stationary wave where destructive interference takes place
Zero amplitude (minimum displacement)
69
Q

What is an antinode? (2)

A

A point on a stationary wave where constructive interference

Maximum displacement/amplitude

70
Q

How are stationary waves formed on stretched strings? (3)

A

A signal generator vibrates the string, forming a wave
The created wave reflects off of the clamp at the end of the string and interferes with the initial wave
Only forms when string is equal to or longer than the wavelength of the wave

71
Q

How are stationary waves formed using microwaves? (2)

A

A microwave produced by a transmitter reflects off of a metal plate
A receiver detects the nodes and antibodies formed

72
Q

What is the phase difference along a stationary wave? (2)

A

In between adjacent nodes all particles are in phase with each other
On different sides of a node the particles are in antiphase (π radians)

73
Q

How are stationary waves formed in an air column? (3)

A

Open tube - antinodes at both ends
One closed end - antinode at open end, node at closed end
Closed tube - nodes at both ends

74
Q

What are the similarities between progressive and standing waves? (2)

A

Comprised of oscillations and vibrations

Particles have a frequency, period of oscillation and amplitude

75
Q

How is energy transfer different between progressive and standing waves? (2)

A

Progressive - in direction of wave

Standing - no net energy transfer

76
Q

How is wavelength different between progressive and standing waves? (2)

A

Progressive - distance between two adjacent points oscillating in phase
Standing - twice the distance between adjacent nodes (original λ)

77
Q

How is phase difference different between progressive and standing waves? (2)

A

Progressive - changes across one complete cycle

Standing - in phase between adjacent nodes, antiphase on the other side

78
Q

How is amplitude different between progressive and standing waves? (2)

A

Progressive - all parts have the same amplitude assuming no energy is lost
Standing - maximum displacement at antinode, zero at node

79
Q

Strings - What is the fundamental (1st harmonic)? (2)

A

1/2 a wavelength

Two nodes

80
Q

Strings - What is the 1st overtone (2nd harmonic)? (4)

A

1 wavelength
3 nodes
Half wavelength of fundamental
Double frequency of fundamental

81
Q

Strings - What is the 2nd overtone (3rd harmonic)? (4)

A

3/2 wavelengths
4 nodes
Third of the wavelength of fundamental
Triple the frequency of fundamental

Physics A Level (23 decks)

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