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Physics > Waves > Flashcards

Waves Flashcards

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1
Q

What is a wave

A

A physical phenomenon that transfers energy through a medium without transfering matter

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2
Q

How do waves travel and transfer energy in a medium

A

Through oscillations

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3
Q

What is a transverse wave

A

Wave in which the direction of oscillation is perpendicular to the direction of wave travel

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4
Q

What is a longitudinal wave

A

Wave in which direction of oscillation is parallel to the direction of wave travel

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5
Q

Define Displacement

A

Distance and direction of a vibrating particle from the equilibrium posituon

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6
Q

Define Amplitude

A

Maximum of a vibrating particle from the equilibrium position

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7
Q

Define Wavelength

A

Distance between two adjacent vibrating particles with same velocity at the same displacement

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8
Q

Define Period

A

Time taken for a particle to complete one oscillation

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9
Q

Define frequency

A

Number of complete oscillations performed per second by a particle

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10
Q

Frequency Formula

A

f = 1 / T

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11
Q

What property of a wave is constant as it travels through a medium

A

Wavelength

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12
Q

What changes as a wave crosses between media

A

Speed and Wavelength

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13
Q

Wave equation

A

v = f x wavelength

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14
Q

Define Phase

A

Fraction of a complete wave that a particle is at

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15
Q

Define Phase Difference

A

The difference in the phase between two points along the same wave or between two waves at any given point in time

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16
Q

Phase difference for two waves in phase

A

Even integer of pi

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17
Q

Phase difference for two waves out of phase

A

Odd integer of pi

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18
Q

Define Reflection

A

When a wave reverses direction upon meeting the boundary between two different medium

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19
Q

Law of Reflection

A

Angle of Incidence = Angle of Reflection

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20
Q

Define Refraction

A

When a wave changes direction upon crossing the boundary between two different media

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21
Q

What does refraction lead to

A

A change of wavelength

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22
Q

What happens as waves slow down

A

Bend towards normal
Wavelength gets shorter

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23
Q

What happens as waves speed up

A

Bend away from normal
Wavelength gets longer

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24
Q

When do sound waves speed up

A

Going into physically denser mediums

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25
Q

When do EM waves slow down

A

Going into optically more dense media

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26
Q

Define Diffraction

A

The physical phenomenon of waves spreading out when passing through a gap, or around an obstacle

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27
Q

Narrower the gap the diffraction is?

A

Greater

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28
Q

Longer the wavelength, the diffraction is?

A

Greater

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29
Q

What must happen for significant diffraction to occur

A

Gap size has to be of same order of magnitude as wavelength

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30
Q

What doesn’t change upon diffraction

A

Wave speed and wavelength

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31
Q

Define Polarisation

A

Property of transverse waves which defines the plane of oscillation of the wave

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32
Q

Plane of polarisation definition for an EM Wave

A

The plane in which the electric field vibrates

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33
Q

What kind of EM waves do most sources of light generate

A

Unpolarised EM Waves

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34
Q

What does unpolarised light consist of

A

Wave-trains within which different waves have their E-field aligned in different planes

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35
Q

2 ways EM waves can be polarised

A

Absorption and reflection

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36
Q

When the plane of polarisation of the incident wave is at some angle to the plane of alignment of the filter, what relationship do we use for intensity

A

Intensity is directly proportional to cos^2 (theta)

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37
Q

What filters do sunglasses have and why

A

Vertically aligned polaroid filters

Block all horizontally plane polarised light

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38
Q

When is a wave polarised

A

When vibrations occur in a single plane

39
Q

When is a transverse wave unpolarised

A

Vibrations change from one plane to another

40
Q

Define Intensity

A

Power per unit area

41
Q

What happens to waves from a point source as they spread out

A

They travel outwards

Intensity falls as power of source is spread over an increasing area

Area over which power is spread = surface area of a sphere of radius equivalent of the distance travelled from the source

42
Q

Intensity at a given distance from the source of power is given by

A

I = P / A

I = P / 4 pi r^2

43
Q

What relationship does intensity of a wave have with distance travelled from source

A

Inverse square relationship

44
Q

Relationship between Intensity and Amplitude

A

Intensity is directly proportional to amplitude squared

45
Q

Wavelength for Radio waves

A

> 10^-1

46
Q

Wavelength for microwaves

A

Between 10^-3 and 10^-1

47
Q

Wavelength for Infrared waves

A

7 x 10^-7 to 10^-3

48
Q

Wavelength for visible waves

A

4 x 10^-7 to 7 x 10 ^-7

49
Q

Wavelength for UV Waves

A

10^-8 to 4 x 10^-7

50
Q

Wavelength for X rays

A

10^-13 to 10^-8

51
Q

Wavelength for gamma rays

A

<10^-13

52
Q

Range of wavelengths where x rays and gamma rays overlap

A

10^-13 to 10^-10

53
Q

When do all EM waves travel at the same speed

A

Only through a vacuum

54
Q

For EM waves how does the electric field vector oscillate

A

90 degrees to the direction of wave travel

55
Q

Define Refractive index

A

ratio between the speed of light in a vacuum to the speed of light in that medium

n = c/v

56
Q

What happens with a greater refractive index

A

Greater the decrease in speed in a medium and the more light refracts in the medium

57
Q

Define TIR

A

Wave phenomenon by which light completely reflects back at a boundary between two media

58
Q

Conditions for TIR

A

Medium within which light is incident has a larger refractive index than the second medium

Angle of incidence exceeds the critical angle

59
Q

Define the principle of superposition

A

When two waves meet, the total displacement at a given point in time is the vector sum of the two individual displacementa

60
Q

What occurs when waves meet in phase

A

Constructive interference

61
Q

What happens when waves meet in antiphase

A

Destructive inteference

62
Q

What is Interference

A

Effect that is observed upon the superposition of waves

Doesn’t always occur when waves superpose

63
Q

When are interference effects observed in practice

A

When two coherent wave sources superpose

64
Q

Define coherent wave sources

A

Wave sources that have the same frequency and constant phase difference at a given point through time

65
Q

What is the P.L.D between two waves

A

Difference in length in the paths travelled by each wave

66
Q

What does PLD give rise to

A

Phase Differences

67
Q

What happens with PLD with an even multiple of wavelength / 2

A

P.D is multiple of 2pi

Constructive interference

68
Q

What happens with PLD with an odd multiple of wavelength / 2

A

P.D is odd multiple of pi

Destructive interference

69
Q

Youngs Double Slit Experiment Method

A

Light from a lamp was passed through a filter

Produces a monochromatic source of light

Monochromatic light was then made incident on a single slit - made to diffract and used to illuminate a double slit - produces two sources of coherent waves

As the light waves from each double slit move forward - they superpose - producing dark and bright interference fringes at a screen - Lights interference pattern

70
Q

Youngs Double Slit Equation

A

only holds true when a &laquo_space;D

wavelength = slit separation x fringe separation / distance between slits and screens

ax/D

71
Q

When do stationary waves form

A

When two progressive waves of the same frequency travel in opposing directions and superpose

For given systems - only certain frequencies travelling along the system will produce stationary waves

72
Q

What is the first harmonic / fundamental frequency

A

Simplest stationary wave that can be produced

Lowest frequency sound that can be produced on a string of a given length, mass and tension

1 antinode in the middle 2 nodes at both ends

L of string = lambda / 2

lambda = 2L

f0 = v /2L

73
Q

What is the second harmonic

A

Second mode of vibration that can be produced

2 antinode in the middle 3 nodes

L = lambda

lambda = L

f1 = v / L

f1 = 2f0

74
Q

What is the third harmonic

A

Third mode of vibration that can be produced

3 antinode in the middle 4 nodes

L = 3lambda / 2

lambda = 2L/3

f2 = 3v/2L

75
Q

What are Photons

A

Packets of discrete EM energy

76
Q

Energy of a photon equation

A

E = hf = hc/lambda

77
Q

What is the UV catastrophe

A

Wave theory can not explain the existence of peaks in intensity at particular wavelengths - as intensity of radiation from an object should become infinite at smaller and smaller wavelengths

78
Q

How did Planck solve the UV Catastrophe

A

Introduced the idea that energy of vibrating atoms can only be in multiples of a basic amount
(quantised)

Introduced Planck’s Constant in E=hf

79
Q

What does a laser beam consist of

A

Photons of the same frequency

80
Q

Power of beam equation

A

nhf

n is the number of photons passing a point every second

81
Q

What is the Electron Volt

A

Unit eV

Work done on/by an electron in accelerating it between a potential difference of 1 volt

V = E/Q
E= VQ
E = Ve (for an electron of e moving across a p.d V)

82
Q

Number of eV equation

A

Energy in J / 1.6 x 10^-19

83
Q

What can be assumed at threshold p.d of a diode

A

Energy of a single electron is transferred completely to the energy of a single photon of a given frequency/wavelength

84
Q

What is the Photoelectric effect

A

The emission of electrons from the surface of a metal, when it is illuminated by EM radiation above a certain threshold frequency

85
Q

Photoelectric Effect Observations

A

Emission of electron only occurs if frequency of incident EM radiation is above threshold

Incident EM radiation above the threshold frequency results in instantaneous emission of electrons

Increasing the intensity of incident radiation increases the number of electrons per second not KE - as long as its above threshold frequency - KE can only be increased by increasing the frequency of incident radiation

86
Q

Photoelectric Effect Explanations

A

A single electron on a metal absorbs the energy carried by a single photon

If the energy carried by a single photon exceeds the work function of the metal - the electron is able to escape the metal

Any energy in excess of the work function absorbed by the electron becomes its kinetic energy

hf = E(kmax) + work function

Emission only takes place is hf > work function

Increasing intensity of incident EM radiation increases number of photons passing per unit time - increasing the number of photon-electron interactions per unit time - increasing the number of emitted electrons

87
Q

Idea of KEmax

A

Relative position of electrons within a metal dictate how much energy is required to free them from a given metal

Electrons on the surface on a metal are subject to fewer electrostatic forces of attraction to ions - and thus require the least amount of energy to free

For any given frequency of incident radiation above threshold - only few, surface electrons acquire max KE

Work function is only for surface electrons

88
Q

When is wave like nature of light observed

A

During diffraction - interference patterns

Light emerging from a narrow slit spreads out - superposes and forms an interference pattern

89
Q

When is particle like nature of light observed

A

Photoelectric effect

Light above a certain threshold frequency is incident on a metal surface - an electron will absorb a single photon

Energy of the photon is a function of the frequency of the light

If the energy of this photon is not greater than the work function of the metal - the electron will not escape

90
Q

What does the de Broglie Wavelength equation tell us

A

Th wavelength of a particle is equal to its momentum

91
Q

De Broglie Equation

A

lambda = h/p

p = particles momentum

Wavelength of a particle depends inversely upon its mass and velocity

92
Q

What happens to de Broglies wavelength when speed increases

A

It decreases

93
Q

Electron Diffraction and Wave Particle Duality

A

Electrons can only be diffracted by metal lattices where the atomic spacing between is comparable to the size of the electron de Broglie Wavelength

Electrons are only diffracted at certain angles - formation of rings

KE and velocity of the electrons can be increased by increasing the accelerating potential between the filament node

Physics (19 decks)

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