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Physics > Section 3 - Waves > Flashcards

Section 3 - Waves Flashcards

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1
Q

What is the definition of frequency and what are the units?

A

The number of waves passing through a point per second (hertz) Hz

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2
Q

What is the definition of wavelength?

A

The distance between 2 adjacent points on a wave

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3
Q

What is the definition of amplitude?

A

The maximum displacement of the wave from its equilibrium position

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4
Q

How can you find out the time period of a wave using its frequency?

A

T = 1/f

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5
Q

What is phase difference and what is it measured in?

A
  • How much a particle/wave lags behind another particle/wave
  • Measured in radians, degrees or fractions of a cycle
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6
Q

What is a longitudinal wave?
Give an example

A
  • A wave in which the oscillation of the particles is parallel to the direction of energy transfer
  • There are rarefractions (areas of low pressure) and compressions (areas of high pressure)
    Sound waves
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7
Q

What is a transverse wave? Give 2 examples

A
  • Waves where the particle oscillations are perpendicular to the direction of energy transfer
    Electromagnetic waves and waves on a string
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8
Q

How fast do electromagentic waves travel in a vacuum?

A

All travel at the speed of light - same speed

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9
Q

What is the relationship between the magnetic field and electric field?

A

Perpendicular

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10
Q

What does a polarising filter do?

A

Only allows oscillations in one plane

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11
Q

How is polarisation used as evidence of the nature of transverse waves?

A

Polarisation can only occur if a wave’s oscillations are perpendicular to its direction of travel (in transverse waves)

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12
Q

How is polarisation used in antennas?

A
  • TV and radio signals are usually plane-polarised by the orientation of the rods on the transmitting aerial
  • The receiving aerial must be aligned in the same plane of polarisation to receive the signal at full strength
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13
Q

What is a stationary wave?

A

A wave which transfers no energy and whose positions of maximum and minimum amplitude are constant

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14
Q

What is a node?

A

A point on a stationary wave where the displacement is 0

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15
Q

What is an antinode?

A

A point on a stationary wave with maximum displacement

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16
Q

How are stationary waves produced?

A

A stationary wave is formed from the superposition of 2 progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude - often a wave reflected back on itself
* Where the waves meet in phase, constructive interference occurs so antinodes form
* Where the waves meet completely out of phase, destructive interfernece occurs and nodes form

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17
Q

What does the first harmonic for a stationary wave with 2 closed end look like?

A

2 nodes at either end and an antinode in the middle

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18
Q

What is the definition of coherence?

A

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

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19
Q

Why is a laser useful in showing interference and diffraction?

A

It produces monochromatic (same wavelength/colour) light so diffraction and interference patterns are more defined

20
Q

What was Young’s double-slit experiment?

A
  • A single light source is directed towards 2 slits - which each act as a coherent light source
  • The light interferes constructively and destructively to create an interference pattern
21
Q

Describe the interference pattern created using white light

A

A bright white central maxima flanked by alternating spectral fringes of decreasing intensity with violet closest to the zero order and red furthest

22
Q

Why does an interfernce pattern form when light is passed through a single slit?

A
  • Light diffracts as it passes through the slit
  • Where the waves are in phase constructive interference occurs making bright fringes
  • Where the waves are completely out of phase destructive interference occurs making a dark fringe
23
Q

How does increasing the slit width change the width of the central maximum?

A

The slit isn’t as close as it was to the wavelength in size so less diffraction occurs - the central maximum becomes narrower and more intense

24
Q

What does a single slit pattern on a graph look like?

A

Central peak and then tiny tiny peaks either side

25
Q

What is the approximate refractive index of air?

A

1

26
Q

When light enters a more optically dense medium does it bend towards or away from the normal?

A

Towards the normal

27
Q

When does total internal reflection occur?

A

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

28
Q

What is the purpose of the cladding in a step index optical fibre?

A
  • Protects core from scratches which would alow light to escape and degrade the signal
  • Allows TIR as it has a lower refractive index than the core
29
Q

How does signal degradation by absorption in an optical fibre affect the received signal?

A

Part of the signal’s energy is absorbed by the fibre so its amplitude is reduced

30
Q

What is pulse broadening?

A

When the received signal is wider than the original, this can cause overlap of signals leading to information loss

31
Q

How does modal dispersion cause pulse broadening?

A
  • Light rays enter the fibre at different angles so they take different paths along it
  • Some may travel down the middle while others are reflected repeatedly
  • The rays take different times to travel along the fibre, causing pulse broadening
32
Q

What is material dispersion?

A

When light with different wavelengths is used some wavelengths slow down more than others in the fibre so they arrive at different times, causing pulse broadening

33
Q

How can modal dispersion be reduced?

A

Use a single mode fibre (very narrow fibre) so the possible differene in path lengths is smaller

34
Q

How can material dispersion be reduced?

A

Use monochromatic light

35
Q

How can both absorption and dispersion be reducced?

A

Use an optical fibre repeater to regenerate the signal

36
Q

State the advantages of optical fibres over traditional copper wires

A
  • Signal can carry more information as light has a high frequency
  • No energy lost as heat
  • No electrical interference
  • Cheaper
  • Very fast
37
Q

What path does a light ray take when the angle of incidence is equal to the critical angle?

A

It goes along the boundary ie the angle of refraction is 90°

38
Q

What is snells law?

A

n1sini = n2sinr

39
Q

What are 2 applications of diffraction gratings?

A
  • Splitting up light from stars to make line absorption spectra - used to identify elements present in the star
  • X-ray crystallography to find the spacing between atoms - a crystal sheet acts as the diffraction grating the X-rays pass through
40
Q

Derive the formula dsinθ = nλ

A
  1. Draw this https://commons.wikimedia.org/wiki/File:Youngs_slits.png
  2. For first order maximum, the path difference between 2 adjacent rays of light is 1λ (as shown), the angle between the normal to the grating and the light ray is θ (the lower angle is 90-θ)
  3. For the first oder maximum sinθ=λ/d (sinθ = opp/hyp) rearrange to dsin = λ
  4. Other maxima occur when the path difference between the 2 rays of light is nλ, where n is an integer, replace λ with nλ to get dsin = nλ
41
Q

When light passing through a diffraction grating is changed from blue to red, do the orders get closer together?

A

The wavelength of light has increased so it will diffract more, the orders will become further apart

42
Q

What is diffraction?

A

The spreading out of waves when they pass through or around a gap

43
Q

How did Young’s double slit experiment provide evidence for the wave nature of light?

A

Diffraction and interfernece are wave properties hence the interference pattern of light shows light has wave properties

44
Q

What are 4 safety precautions that must be followed when using a laser?

A
  • Wear laser safety goggles
  • Don’t shine the laser at reflective surfaces
  • Display a warning sign
  • Never shine the laser at a person
45
Q

How could you investigate stationary sound waves?

A
  • Place a speaker at one end of a closed glass tube
  • Lay powder across the bottom of the tube
  • It will be shaken from the antinodes and settle at the nodes
  • The distance between each node is half a wavelength
46
Q

What is phase?

A

The position of a certain point on a wave cycle, (units are radians, degrees or fractions of a cycle)

Physics (14 decks)

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