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Physics A Level > Waves > Flashcards

Waves Flashcards

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1
Q

What is Displacement

A

The distance and direction from equilibrium position of a particle on a wave

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2
Q

What is wavelength?

A

Least distance between 2 adjacent vibrating particles with same phase

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3
Q

What is a complete cycle?

A

The max displacement to next max displacement

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4
Q

What is time period?

A

Time for 1 wave to pass a fixed point (time for 1 cycle)

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5
Q

What is frequency?

A

No of cycles per second

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6
Q

What is phase difference?

A

The difference by which 1 wave leads or follows another

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7
Q

What are progressive waves?

A

Waves which transfers energy from one place to another via oscillations

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8
Q

What is intensity?

A

The rate at which energy is transferred from one location to another
I=P/A
If spread out equally in all directions
I=P/4πr²

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9
Q

What is the relationship of energy and amplitude? ∴ what is the relationship of I and E?

A

E ∝ A²

∴ I ∝ A² ∵ I =P/A and P = E/T

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10
Q

What is interference?

A

Superposition of 2 or more waves

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11
Q

What can interference cause?

A

Change in amplitude of a the wave

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12
Q

What can all em waves do?

A
  • travel through a vacuum
  • speed of light
  • transverse
  • can be reflected, refracted and diffracted
  • can demonstrate interference
  • can be polarised
  • possess a magnetic wave and an electrical wave interlocked + oscillation @right angles to each other + direction of travel
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13
Q

Why can’t EM from R-V ionise?

A

Because they do t have enough photon energy

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14
Q

What is the plane of an EM wave?

A

The plane of oscillation of the electric field.

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15
Q

What is a plane-polarised wave?

A

A wave that has oscillations/fields only in one plane only

Perpendicular to direction of E transfer

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16
Q

Why can’t longitudinal waves be polarised?

A

They don’t have oscillations at right angles to their direction of travel.

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17
Q

What is a polarising filter?

A

It produces plane-polarised light by selective absorption of one component of the incident oscillations - the filter transmits only the component of light polarised perpendicular to that direction

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18
Q

What is crossed polaroids?

A

When the analyser is at right angle to the polariser = no light will pass through

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19
Q

What is Malus’ law?

A

When a perfect polariser is put in a beam of polarised light

I=ImaxCos²θ

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20
Q

Describe refraction

A

Occurs when a wave enters from one transparent medium to another where the wave will change speed and direction (unless it’s travelling along the normal)

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21
Q

What happens if the angle of incidence is lower than the critical angle?

A

Refraction will occur and a weak reflected ray

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22
Q

What happens if the angle of incidence is equal to the critical angle?

A

Angle of refraction = 90 degrees

Reflection

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23
Q

What happens if the angle of incidence is greater than the critical angle?

A

No refraction

Total internal reflection

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24
Q

What is the principle of superposition?

A

States - When two or more waves of the same type meet, the resultant wave can be found by adding the displacements of the individual waves

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25
Q

What is constructive interference?

A

When 2 waves in phase superpose to make a wave w/ a larger displacement

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26
Q

What’s destructive interference?

A

When 2 waves in anti-phase superpose to make a wave w/ reduced displacement

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27
Q

What is coherence?

A

2 waves with a constant phase relationship w/ same wavelength and hz

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28
Q

What’s in phase?

A

Same velocity

& same hz & wavelength

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29
Q

What is needed for constructive interference to take place?

A

Path difference must be a whole number + waves be in phase

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30
Q

What is needed for destructive interference to take place?

A

Waves arrive out of phase + path difference of half a wavelength

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31
Q

Why is it difficult to determine the wavelength accurately w/young double slit? How is this overcome?

A

Diffraction fringes are quite blurred ∴ measuring fringe width difficult. Using multiple slits - diffraction grating

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32
Q

What does increasing the no.of slits do?

A

Improves brightness + sharpness + easier to measure an accurate value for wavelength and angle of maxima

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33
Q

What is the photoelectric effect?

A

When photoelectrons are released when em radiation of a particular Hz is shone on the surface of a metal

34
Q

What is the threshold frequency?

A

The lowest frequency of radiation that will result in the emission of electrons from a particular metal surface

35
Q

What is the work function?

A

= min energy required to release an electron from its surface, overcoming the electrostatic attraction between the electron and the positive metal ions

36
Q

What happens if the incident radiation < f0 (threshold frequency)

A

Photons don’t have enough energy to overcome the work function

37
Q

What does work function =

A

Hf0

38
Q

What happens if the single slit is too wide?

A

The contrast will be lost between the dark and bright fringes

39
Q

Describe how a stationary wave is produced

A
  • a strong is tied at one end to a mechanical vibrator connected to a frequency generator
  • the other end = attached to a pulley + supports a weight = tension in string
  • as Hz is increased, different stationary patterns are seen on the string
  • nodes and anti nodes
40
Q

How can the pitch/frequency on a stationary wave be changed?

A

Changing the tension or length of string

Large tension/short length = increases pitch

41
Q

What do you measure for the wavelength for the greatest accuracy?

A

From on max to the furthest one from it

42
Q

What is quantum?

A

Small discrete unit of energy

43
Q

What is photon?

A

Quantum associated with electromagnetic radiation

44
Q

What is the total energy transferred by an EM wave = ?

A

The sum of energies of the photons emitted by the light source

45
Q

Describe how you find Planck’s constant part (1)

A
  1. Increase pd across LED by adjusting variable resistor + record current
  2. Record 6 values for V + I in steps of 0.05V
  3. Plot a graph of I against V for each LED
  4. Activation voltage can be obtained by extrapolating the straight line till it reaches the x axis
  5. Repeat measurements several times for each LED
46
Q

Describe how you find Planck’s constant part (2)

A
  1. Plot a graph of activation voltage against 1/wavelength
  2. eV = hc/wavelength
  3. Gradient = hc/e
  4. ∴ h = gradient * e/c
47
Q

What is the wave particle duality?

A

The idea that objects can exhibit both wave and particle properties

48
Q

Why can’t we use diffraction gratings for electrons?

A

The wavelength is too small

49
Q

What do we use instead of diffraction gratings for electrons?

A

A carbon disc - they diffract as they emerge from the gaps between atomic layers

50
Q

What pattern do you get in graphite atoms + why?

A

Circular pattern ∵ graphite atoms = not lined up in the same direction as in a diffraction grating

51
Q

What does de Broglie’s equation predict?

A

That the wavelength of the electron can by changed by varying its velocity

52
Q

How does the velocity affect the wavelength of electrons?

A

As velocity increased, wavelength decrease ∴ diffraction pattern = bigger

53
Q

What are the uses of electron diffraction?

A
  • used to determine atomic spacing

∵ increasing speed = decrease in wavelength -> smaller values of inter atomic spacing can be measured

54
Q

What is an electronvolt?

A

The eK gained by an electron when accelerated through a pd of 1V

55
Q

What happens when the electroscope is charged?

A

The leaf is repelled from the stem

56
Q

How do you charge an electroscope?

A
  • -ve charged polythene rod

* +ve charged perspex rod

57
Q

What happens when you charge by contact?

A

Charge produced = charge on rod

58
Q

What happens when you charge by induction?

A

Charge produced = opposite to charge on rod

59
Q

What will happen if the plate is positively charged?

A

No light source has an effect

60
Q

What will happen if the plate is negatively charged?

A

Visible light has no effect

UV -> leaf falls

61
Q

What does the gold leaf electroscope show?

A

UV light causes metal plate to lose its negative charge through emission of electrons which are repelled by the negative charge on the electroscope

62
Q

Why won’t the effect work if the electroscope is positively charged?

A

Any electrons will be attracts back to the plate

63
Q

Why does the Zn have to be cleaned before the test?

A

Zn may be covered in ZnO ∵ reacted with air

64
Q

Why should the electroscope be locked in a vacuum?

A

∵ ions in the air don’t cause discharge of the electroscope

65
Q

How do you increase the eK of electrons emitted?

A

Increase frequency

66
Q

What happens if the incident radiation is = f0?

A

Electrons will emit BUT with 0 eK

67
Q

Why doesn’t wave intensity affect the eK?

A

It ejects more electrons

68
Q

What is required for TIR?

A

I > C

Wave to be travelling from a material of higher refractive index to lower

69
Q

Why is it better to have coherent waves interfering?

A

Clearer patterns of constructive/destructive can be seen

70
Q

What determines interference to be constructive or destructive?

A

Path difference

71
Q

What is the fundamental mode of vibration?

A

Lowest possible resonant frequency

Describe it - node on each end

72
Q

What is a stationary wave?

A

The superposition of 2 progressive, coherent waves moving in opposite directions
Formed at resonant frequencies

73
Q

How do stationary waves form generally

A

Incident wave is reflected
Reflected wave interferes/superpose with incident wave
Resultant wave w/nodes and antibodies

74
Q

How is the work function related to the threshold frequency?

A

A photon < f0 cannot cause electron emission ∴ work function = hf0

75
Q

Why is there a maximum value for kinetic energy of emitted electrons?

A

e- in the metal have a range of energy is

most require more than WF energy to escape from the surface

76
Q

Why can’t we use two separate identical light sources in the double slit experiment?

A

Light from two sources must be coherent – only possible to produce Constant phase difference using a single source

77
Q

Explain the nature of interference that occurs at the origin point in the double slit

A

Constructive interference due to path difference equalling 0/Waves meet in phase

78
Q

Why does the amplitude of the detected signal change when the detector is moved back from 2 microwave transmitters

A

The path difference between signals changes causing detected signal to vary between maximum and minimum
Max when in phase and min when out of phase

79
Q

Why does the reflective wave have a different amplitude?

A

Reflected wave is weaker/reflective wave has travelled a greater distance

80
Q

State and explain the conditions necessary for light From two slits to produce a visible pattern on the screen

A

Light must be coherent i.e. constant state relationship and similar amplitudes
Slits must also be narrow so diffraction patterns overlap

81
Q

What’s the difference between an absorption line spec and an emission line spec

A

In an absorption spectrum a series of dark lines appear against a bright background
In emission there’s a series of bright lines against a dark background

Physics A Level (14 decks)

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