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Physics > Waves (Unit 2) > Flashcards

Waves (Unit 2) Flashcards

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1
Q

Longitudinal wave

A

Particle vibration is parallel to direction of wave propagation

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2
Q

Examples of a longitudinal wave

A

Sound waves, seismic p-waves

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3
Q

Transverse wave

A

Particle vibration is perpendicular to direction of wave propagation
Only transverse waves can be polarised

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4
Q

Examples of a transverse wave

A

Electromagnetic radiation, seismic s-waves

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5
Q

Particle displacement

A

The distance of a particle from its equilibrium position in given direction

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6
Q

Amplitude

A

the maximum displacement of a particle (wave) from its equilibrium (or rest) position

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7
Q

Frequency

A

Number of oscillations (of a particle) per second

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8
Q

Time period

A

The time for one complete oscillation

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9
Q

Wavelength

A

Shortest distance between two points in phase

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10
Q

Diffraction

A

Spreading out of a wave (when it passes through a gap or past the edge of an object)

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11
Q

Refraction

A

Wave bends/changes direction when its speed changes

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12
Q

Polarisation

A

(transverse) wave oscillation is in one plane

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13
Q

Application of polarisation in sunglasses

A
  • Light reflected from surfaces is (weakly) polarised in one plane (horizontal)
  • Polaroid in sunglasses can be orientated to remove this reflected light
  • Reducing glare
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14
Q

Application of polarisation in tv transmitters and aerials

A
  • Signals from tv transmitter (radio waves) are polarised
  • Aerials need to be orientated (rotated) so they are in same plane as the transmitted signal
  • For maximum signal strength
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15
Q

Superposition

A

Where two or more waves meet, the resultant displacement equals the vector sum of the individual displacements

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16
Q

Conditions for formation of stationary waves

A
  • Two waves travelling past each other in opposite directions
  • With the same wavelength (or frequency)
  • Similar amplitudes
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17
Q

Nodes and antinodes

A

Nodes – points of no oscillation / zero amplitude

Antinodes – points of maximum amplitude

18
Q

Coherent sources

A

waves (from two sources) that have:
• a constant phase difference
• same wavelength (or frequency)

19
Q

Monochromatic

A

Single wavelength

20
Q

Safety with a laser

A
  • Avoid looking along the beam of a laser
  • Wear laser safety goggles
  • Avoid reflections
  • Put up a warning sign that a laser is in use
21
Q

Properties of laser light

A
  • Monochromatic – only a single wavelength
  • Coherent – waves have a constant phase difference
  • Collimated – produces an approximately parallel beam
22
Q

Appearance of interference fringes from two vertical slit illuminated with yellow light

A
  • Vertical or parallel
  • Equally spaced
  • Black and yellow bands
23
Q

Fringe width, w, changes

A

Slits closer together w – increases
Screen further away w – increases
Shorter wavelength (eg blue light) w - decreases

24
Q

Explanation of formation of fringes with Young’s slits

A
  • Interference fringes formed
  • Where light from two slits overlaps
  • The light from the two slits is coherent
  • Bright fringes formed where constructive interference
  • because light from the two slits is in phase (path difference equals a whole number of wavelengths)
  • Dark fringes formed where destructive interference
  • Because light from the two slits is in anti-phase (path difference equals a whole number + 0.5 wavelengths)
25
Q

Appearance of white light through Young’s slits

A
  • Central fringe would be white
  • Side fringes are (continuous) spectra
  • Bright fringe would be blue on the side nearest the central fringe.
  • Bright fringes merge further away from centre.
26
Q

appearance of diffraction pattern from a single slit

A
  • Central bright fringe has twice width of other bright fringes
  • The other bright fringes have a much lower intensity
  • and are equally spaced
27
Q

Single slit pattern changes

A

Narrower slit width • Wider pattern / increased separation
• Reduced intensity
Shorter wavelength • Narrower pattern / reduced separation

28
Q

Lines per mm of a grating

A

Spacing, d, of slits on a diffraction grating given by:

d = 1/(number of lines per mm) in mm

29
Q

Applications of gratings to spectral analysis of light from stars

A
  • Dark lines in spectrum from a star (absorption spectrum)

* Reveal the composition of (elements present in) the star’s atmosphere

30
Q

How does light change moving from air to glass

A
  • speed – decreases (slows down)
  • wavelength – decreases (gets shorter)
  • frequency – remains constant (stays the same)
31
Q

Conditions for total internal reflection

A
  • Angle of incidence is greater than the critical angle
  • The refractive index of the material light is going from is greater than the refractive index of the material the light is going to.
32
Q

Total internal reflection

A

Where all the light is reflected back into the material

33
Q

Critical angle

A

Angle of incidence which produces an angle of refraction of 90 degrees.

34
Q

Structure of an optical fibre

A

Central core, surrounded by cladding. Refractive index of core must be greater than refractive index of cladding (to ensure total internal reflection)

35
Q

Purpose of cladding

A
  • prevents crossover of signal/data to other fibres
  • prevents scratching of the core
  • reduces pulse broadening/dispersion
36
Q

Use of optical fibres

A
  • Communication – improve transmission of data/high speed internet
  • Endoscopes – improved medical diagnosis
37
Q

How do pulses of light change travelling down optical fibres

A
  • reduced amplitude due to absorption/energy loss and scattering within fibre
  • pulse broadening due to multipath dispersion from rays taking different paths and different times to travel down same fibre
38
Q

How is multipath dispersion reduced

A

Core of fibre is made very narrow/thin.

39
Q

Sketches of stationary waves for first 4 harmonics

A

See sheet

40
Q

Derivation of n(lambda) = d sin(theta)

A

See sheet

Physics (12 decks)

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