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A-Level Physics > 3: Waves > Flashcards

3: Waves Flashcards

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1
Q

Progressive Wave

A

Waves, which transfer energy

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2
Q

Amplitude

A

The maximum displacement of a point on a wave from its rest position

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3
Q

Frequency

A

The amount of oscillations completed by a point on a wave per unit time

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4
Q

Wavelength

A

The length between two of the exact same points on a wave

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5
Q

Wave Speed

A

The speed at which waves travel

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6
Q

Wave Speed Formula

A

c = f λ

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7
Q

Frequency Formula

A

f = 1 / T

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8
Q

Longitudinal Waves

A

Waves where the direction of displacement of oscillating particles / fields is parallel to the direction of energy propagation

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9
Q

Transverse Waves

A

Waves where the direction of displacement of oscillating particles / fields is perpendicular to the direction of energy propagation

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10
Q

Phase

A

The fraction of a cycle that a point on a wave has completed since the start of that cycle

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11
Q

Phase Difference

A

Difference in phase between the exact same point on two waves measured in degrees, radians or fractions of a cycle

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12
Q

Polarisation

A

Where transverse waves are all oriented in the same direction – oscillating in the same plane (for EM waves, it is the electric fields, which can be polarised)

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13
Q

Applications of Polarisers (2)

A
  • Polaroid material
  • Alignment of aerials for transmission and reception
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14
Q

Polaroid material

A

A material, which polarises transverse waves (e.g., sunglasses)

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15
Q

Alignment of Aerials for Transmission & Reception (4)

A
  1. AC in transmitting aerial oscillates electromagnetic fields in one direction
  2. This produces a polarised EM waves
  3. EM waves oscillate electrons in receiving aerial producing the same frequency AC
  4. Transmitting and receiving aerials have to be aligned in the same orientation so that the waves are fully absorbed
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16
Q

Refractive Index of a Substance s

A

n = c / cₛ
n is refractive index of s
c is speed of light in vacuo in m s⁻¹
c is speed of light in s in m s⁻¹

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17
Q

Refractive Index of Air

A

~1

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18
Q

Refractive Index between Two Boundaries

A

₁n₂ = n₂ / n₁
₁n₂ is relative refractive index of a boundary (material 1 to material 2)
n₁ is refractive index of material 1
n₂ is refractive index of material 2

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19
Q

Snell’s Law of Refraction for a Boundary

A

n₁ sin θ₁ = n₂ sin θ₂
n₁ & n₂ are refractive indexes of materials 1 and 2
θ₁ is angle of incidence in °
θ₂ is angle of refraction in °

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20
Q

Total Internal Reflection (3)

A

When a ray travels from a more optically dense medium to a less, there will be a critical angle, θ. If:
- θᵢ < θ, there’s refraction and partial reflection
- θᵢ = θ, there’s refraction at 90° and partial reflection
- θᵢ > θ, there’s total reflection

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21
Q

Critical Angle Formula

A

sin θ = n₂ / n₁ for n₁ > n₂
θ is critical angle of boundary in °
n₁ & n₂ are refractive indexes of materials 1 and 2

22
Q

Polarisation is Evidence for ____

A

The nature of transverse waves

23
Q

Optical Fibres

A

A very thin tube of glass or plastic fibre, that can carry signals over long distances and round corners using total internal reflection

24
Q

Step-Index Optical Fibres

A

Have a high refractive index core surrounded by low refractive index cladding to allow total internal reflection, which prevents light from escaping. The cladding also protects the core from scratches, which could allow light to escape

25
Q

Absorption in Fibre Optics

A

Some of the pulse’s energy is absorbed by the material, which reduces the pulse’s amplitude, meaning the receiver may not be able to pick up the signal

26
Q

Pulse Broadening in Fibre Optics

A

There are two types of dispersion – modal and material. They both result in pulse broadening – where the pulse spreads out as it travels through the core. Broadened pulses can overlap leading to information loss

27
Q

Modal Dispersion (3)

A
  • Caused by the light rays entering the optical fibre at different angles. This means they take different paths with varying distances down the fibre so there is a wide range of times at which the rays arrive at the receiver resulting in pulse broadening
  • Can be reduced by using cladding with a higher refractive index, which lets rays with small angles of incidence (on the boundary between the core and cladding) escape the core
  • Can be reduced by narrowing the core so there’s less variation in the angles of incidences of the rays
28
Q

Material Dispersion (2)

A
  • Given that light consists of different wavelengths, different rays experience different amounts of refraction so they slow down by different amounts. Thus, there’s a range of times where they reach the receiver resulting in pulse broadening
  • Using monochromatic (all one wavelength) light can stop material dispersion
29
Q

Optical Fibre Repeaters

A

Regenerate the signal at regular intervals, which reduces signal degradation

30
Q

Principle of Superposition of Waves

A

When two or more waves cross, the resultant displacement equals the vector sum of the individual displacements

31
Q

Path Difference (3)

A
  • The difference, in wavelengths, between the distances of the paths of two waves to a point
  • If it is an integer (0, λ, 2λ) the waves superpose to produce a maximum
  • If it is an integer plus 1/2 (λ/2, 3λ/2), the waves cancel each other out
32
Q

Coherence

A

Where waves have the same wavelength and frequency and a fixed phase difference

33
Q

Stationary Waves

A

The superposition of two progressive waves, with the same frequency, wavelength and amplitude, travelling in opposite directions

34
Q

Nodes

A

Points on the stationary wave where the progressive waves always superpose to give 0 amplitude / displacement as they have a phase difference of π radians

35
Q

Antinodes

A

Points on the stationary wave where the progressive waves superpose to produce an oscillation between maximum positive and negative displacements (progressive waves are always in phase)

36
Q

Resonant Frequencies (1, 1:3, 1)

A
  • A stationary wave is only formed at a resonant frequency (when an exact number of half wavelengths fits on the string)
  • First harmonic:
    • Lowest possible resonant frequency
    • Wave has one “loop” with a node at each end and an antinode in the middle
    • 1/2 wavelength fits on string
  • Second harmonic is twice the frequency of the first harmonic
37
Q

First Harmonic Equation

A

f = (1 / 2l) √(T / μ)
f is frequency in Hz
l is length in m
T is tension in N
μ is mass per unit length in kg m⁻¹

38
Q

Young’s Double Slit Experiment

A

The use of two coherent sources or the use of a single source with double slits to produce an interference pattern

39
Q

Fringe Spacing Equation

A

w = λ D / s
w is fringe spacing in m
λ is wavelength in m
D is distance between slits and screen in m
s is distance between slits in m

40
Q

Diffraction Grating Equation

A

d sin θ = n λ
d is distance between slits in m
θ is angle to the normal made by the maximum
n is order of maximum
λ is wavelength of light source in m

41
Q

Derivation of d sin θ = n λ (5)

A
  • https://th.bing.com/th/id/OIP.FPwz0Tf_bHcdHpVxKEoe8QAAAA?pid=ImgDet&rs=1
  • The angle between the nth order maximum and incoming light is θ
  • d is slit spacing
  • n λ is the path difference at the nth order maximum
  • sin θ = n λ / d
    → d sin θ = n λ
42
Q

Diffraction Grating

A

Diffraction gratings contain equally spaced slits. The interference pattern produced using monochromatic light is really sharp as there are lots of rays reinforcing the pattern

43
Q

Applications of Diffraction Gratings (2)

A
  • Astronomers and chemists analyse the spectra of stars and materials respectively, produced by a diffraction grating, to see what elements are present
  • Crystals can be used as diffraction gratings for X-rays (which have a similar wavelength to the crystals atom separation) and the interference pattern gives information about the crystal structure
44
Q

Required Practical 1

A

Investigation into the variation of the frequency of stationary waves on a string with length, tension and mass per unit length of the string

45
Q

Required Practical 1 Method (4)

A

https://docs.google.com/document/d/1dnOyhtbTycL6i5XJ8f8RwxPVPK0cZC-GfugFbyWu4tU/edit?usp=sharing
1. Set the length of the string to a known distance
2. Measure the mass of the hook
3. Adjust the frequency on the vibrator until the wave on the string is the first harmonic. Read the frequency
4. Repeat step 4, incrementing the mass on the hook

46
Q

Required Practical 2

A

Investigation of interference effects to include the Young’s slit experiment and interference by a diffraction grating

47
Q

Required Practical 2: Young’s Slit Experiment Method (5)

A
  1. https://docs.google.com/document/d/1MzsvzAsm11bKLadcWOdFqeUHiMMljszp2DKmvPQgcuA/edit?usp=sharing
  2. Read the slit spacing of the double slit
  3. Set a known distance between the double slit and screen
  4. The fringe width can be measured by measuring across a large number of visible fringes
  5. Repeat step 4, changing the distance between the slits and screen
48
Q

Required Practical 2: Interference by a Diffraction Grating Method (2)

A

https://docs.google.com/document/d/1JB0MDui0cTgAxu9-LY8g2XVcII4Pp4FKdq-gX_BFUWI/edit?usp=sharing
1. Record the slit spacing of the diffraction grating
2. Calculate θ₁, θ₂ and θ₃ by measuring h₁, h₂ and h₃ and using the distance between the screen and slits and note the maximum order for each angle

49
Q

Appearance of Single-Slit Diffraction Pattern using Monochromatic Light (2)

A
  • High-intensity central maximum with low-intensity subsidiary maxima on either side
  • Width of central maximum is double the width of other maxima
50
Q

Appearance of Single-Slit Diffraction Pattern using White Light (2)

A
  • High-intensity white central maximum with low-intensity spectra for subsidiary maxima on either side
  • Width of central maximum is double the width of the subsidiary maxima
51
Q

Variation of Width of Central Diffraction Maximum with Wavelength

A

Increasing wavelength increases the width of the central maximum

52
Q

Variation of Width of Central Diffraction Maximum with Slit Width

A

Decreasing slit width increases the width of the central maximum

A-Level Physics (9 decks)

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