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Physics > Waves and Optics > Flashcards

Waves and Optics Flashcards

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1
Q

Effect of using laser with shorter wavelength in Young’s Double Slit Experiment

A
  • quote formula
  • fringe separation is proportional to wavelength
  • shorter wavelength means spacing between maxima decreases
  • as path difference is smaller
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2
Q

Monochromatic

A

single wavelength

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3
Q

Coherent

A
  • same frequency/wavelength
  • constant phase difference
  • NOT IN PHASE
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4
Q

Effect of narrower slit in Young’s Double Slit Experiment

A
  • wider fringe separation

- lower intensity

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5
Q

Pattern on screen for white light in Young’s Double Slit Experiment

A
  • distinct fringes shown with subsidiary maxima
  • entral fringe white
  • blue on inner edge
  • red on outer edge
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6
Q

Describe and explain stationary wave experiment

A
  • waves reflected
  • pass through each other in opposite directions
  • have same wavelength/frequency + similar amplitude - superposition of waves results in cancellation at nodes
  • reinforcement at antinodes
  • energy is not transferred along string
  • nodes have no displacement/zero amplitude
  • antinodes have maximum amplitude
  • between node and antinode amplitude increases
  • particles with an odd no. of nodes between them are 180 degrees out of phase
  • particles with an even no. of nodes between them are in phase
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7
Q

Explain why amplitude of waves is reduced during transmission

A
  • absorbed by medium
  • scattering
  • reflected at different angles by medium
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8
Q

Describe and explain Young’s double slit experiment

A
  • measure w (fringe separation) using vernier calliper
  • measure D (distance between laser and screen) using metre rule
  • ensuring large enough value for D, e.g. 2m
  • use fringe separation formula to calculate wavelength
  • measure across several fringes from CENTRE
  • carried out in dark room (for clearer observation)
  • repeats
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9
Q

Young’s Fringes white light vs red laser light

A
  • different colours vs monochromatic
  • less intense vs more intense
  • maxima closer together for white compared to red laser
  • fringes for white compared to dots for laser
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10
Q

Describe diffraction grating experiment and suggest ways to reduce uncertainties

A
  • measure angle of first order beam using a protractor
    or tan(angle)= w/D as it is easier to measure fringe separation with vernier calliper
  • d = 1/lines per mm
  • measure angle for more than one order (NOT AVERAGE ANGLE but average wavelength)
  • repeat for different/larger distances from screen
  • protractor with 0.5 degrees intervals or less
  • plot n against sin(angle) where gradient =d/wavelength
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11
Q

Describe motion of particles on standing wave (not including at nodes)

A
  • oscillate continuously up and down

- about equilibrium position

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12
Q

Explain importance of correctly aligning aerial of a TV to ensure strongest signal

A
  • transmitted radio waves are often polarised

- aerials must be aligned in same plane of polarisation on waves

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13
Q

Applications of diffraction gratings

A
  • spectrometers (analysis of gas compositions)

- analysis of chemical composition of stars (produces light spectrum)

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14
Q

Purpose of narrow single slit in Young’s Double Slit Experiment

A
  • narrow slit gives wide diffraction
  • ensures both slits are illuminated
  • provides coherent source of light (same wavelength and constant phase difference)
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15
Q

Describe how to obtain an accurate value for wavelength in Young’s Double Slit Experiment

A
  • increase D
  • as this increases w
  • measure across more than 2 maxima
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16
Q

Describe how to determine the maximum number of orders for a transmission diffraction grating

A
  • substitute 90 degree into n x wavelength=dsin(angle)

- n=d/wavelength

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17
Q

Transverse

A

Waves with a direction of oscillation perpendicular to direction of propagation

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18
Q

Longitudinal

A

Waves with a direction of oscillation parallel to the direction of propagation

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19
Q

Polarised vs unpolarised light

A

Photons oscillate in one plane only vs all planes

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20
Q

Describe how to calculate width of central fringe in Young’s Double Slit Experiment

A
  • 2 x wavelength x D / a
  • D (distance between slit and screen)
  • a (slit width)
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21
Q

Waves

A

Periodic disturbances in a material or space

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22
Q

Mechanical waves

A

Waves which travel through a physical medium by means of vibrations

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23
Q

EM waves

A

A photon consisting of electric and magnetic waves in phase travelling at right angles

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24
Q

Polarisation

A

When transverse waves are made to oscillate in one plane only after travelling through a polaroid filter

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25
Q

Displacement

A

Distance and direction of oscillating particle from its equilibrium

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26
Q

Amplitude

A

Maximum displacement from equilibrium position

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27
Q

Wavelength

A

Least distance between two successive oscillating particles that are in phase

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28
Q

Period

A

Time for one complete wave to pass a point

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29
Q

Frequency

A

Number of complete waves passing a point each second

30
Q

Phase difference

A

Fraction of a cycle between two oscillating particles measured in radians or degrees

31
Q

Wavefront

A

Lines of constant phase eg crests

32
Q

Ways to increase diffraction

A
  • narrower gap

- longer wavelength

33
Q

Superposition

A

Effect of adding together two waves together - total displacement equal sum of individual displacements

34
Q

Interference

A

Formation of points of cancellation and reinforcement where two coherent waves meet

35
Q

Explain why intensity of fringes decreases further away from central fringe

A
  • intensity is inversely proportional to distance between slits and screen
  • light travels longer distance further away from central fringe
36
Q

Suggest why you would use monochromatic light in an optical fibre

A
  • prevent spectral dispersion
  • causing signals to merge
  • since different wavelengths at different speeds
37
Q

Stationary Waves

A
  • waves produced when two or more waves
  • similar amplitude and same frequency/wavelength
  • pass through each other in opposite directions
38
Q

Progressive Waves

A

Waves which travel through a substance or space

39
Q

Compare the nature of stationary and progressive waves

A

Stationary
- all particles except those at nodes vibrate at same frequency
- amplitudes varies from zero to maximum at antinodes
Progressive
- all particles vibrate at same frequency
- amplitude is constant

40
Q

Suggest why waves that produce a standing wave do not cancel each other out completely and how complete cancellation could be achieved

A
  • not always 180 degrees out of phase
  • travel in opposite directions in standing wave
  • cancellation only at nodes
  • if waves travelled in the same direction with a phase difference of 180 degrees they would cancel out
41
Q

Suggest how the amplitude of the wave that produces a standing wave compares to amplitude at antinode

A

Amplitude at antinode is double original amplitude since complete superposition occurs

42
Q

Suggest how the properties of an EM wave change when it travels through a denser medium

A
  • frequency constant always

- wavelength and speed change

43
Q

Explain how fringes are formed in Young’s Double Slit Experiment

A
  • slits act as coherent sources of light
  • waves diffract at slits
  • waves overlap and interfere
  • bright patches = reinforcement when the path diff is a whole number of wavelengths
  • dark patches = cancellation when the path diff is an odd number of half wavelengths
44
Q

Differences between transverse and longitudinal waves

A
  • difference between direction of oscillation and propagation
  • longitudinal all require a medium to travel through
  • only transverse can be polarised
45
Q

Suggest why longitudinal waves cannot be polarised

A

Particles oscillate in the same direction as wave propagates

46
Q

Examples of transverse and longitudinal waves

A 
Transverse 
- EM waves 
- waves on string 
- water waves 
- secondary seismic waves 
Longitudinal 
- sound waves
- primary seismic waves
47
Q

Suggest why TIR does not take place when light travels from water (n=1.33) to glass (n=1.47)

A

TIR only takes place when light travels from a material with higher to lower refractive index

48
Q

Suggest why cladding is used in optical fibres

A
  • lower refractive index (step index) so ensures TIR reduces refraction out of core (signal loss)
  • prevents scratching of core / breakage
  • prevents crossover of signals to other fibres
  • increases tensile strength since core needs to be thin
49
Q

Suggest why stationary waves only occur at specific frequencies

A
  • at frequencies other than a harmonic frequency
  • interference of reflected and incident waves
  • results in irregular and non-repeating disturbance in medium
50
Q

Node

A

no displacement/zero amplitude

51
Q

Antinode

A

maximum amplitude

52
Q

Path Difference

A

difference in distance travelled by two coherent waves from two different sources at a particular point (given in metres or in terms of wavelength)

53
Q

Diffraction

A

spreading of waves on passing through a gap or near an edge

54
Q

Describe the relationship between width of central fringe and slit width/distance between slit and screen in Young’s Double Slit Experiment

A
  • directly proportional to distance between slit and screen

- inversely proportional to slit width

55
Q

Explain why light transmitted by diffraction grating is in certain directions only

A
  • light passing through slit is diffracted
  • diffracted waves from adjacent slit reinforce each other in certain directions only
  • cancel out in all other directions
56
Q

Suggest why diffraction grating is useful in separating colours in incident light

A
  • different wavelengths diffracted at different angles

- according to n x wavelength=dsin(angle)

57
Q

Suggest how to calculate grating spacing given lines per mm

A

d = 1/lines per mm

convert to m from mm

58
Q

Explain the derivation of diffraction grating formula given by n x wavelength=dsin(angle)

A
  • wavefront emerging from slit P reinforces wavefront from adjacent slit Q
  • earlier wavefront from Q must have travelled n wavelengths from slit
  • hence distance QY from slit to wavefront is n x wavelength
  • d is distance PQ
  • angle between wavefront and plane of slits given by sin(angle)=QY/QP
  • hence sin(angle) = (n x wavelength) / d
59
Q

Approximate refraction index of air

A

1

60
Q

Harmonics

A

Integer (whole number) multiples of fundamental frequency

fn = nf, where n is harmonic number and f is fundamental frequency (wavelength/2)

61
Q

Fundamental Frequency

A
  • lowest frequency of periodic waveform
  • longest wavelength
  • wavelength/2
62
Q

Conditions for total internal reflection

A
  • refractive index of first medium is greater than refractive index of second medium
  • angle of incidence is greater than critical angle
63
Q

Modal/Multipath Dispersion

A
  • lengthening of light signal pulse as it travels along optical fibre
  • light enters optical fibre at different angles so different TIR
  • rays that repeatedly undergo total internal reflection having to travel longer distances compared to those that undergo fewer
    =pulse broadening
64
Q

Material Dispersion

A
  • lengthening of a light signal as it travels along an optical fibre
  • monochromatic light contains a continuum of wavelengths within a small range
  • wavelengths with a higher refractive index are totally internally reflected more so pulse take longer to travel down fibre
    = pulse broadening
65
Q

Pulse broadening

A
  • spreading of light pulses as they travel down optical fibres
  • merging of signals
  • poor quality signal/signal loss
  • slower transmission
66
Q

Suggest why a narrow core is used in optical fibres

A
  • increased probability of TIR so less light lost by refraction out of core
  • reduces number of waves entering fibre at high angle of incidence to reduce multipath dispersion
  • waves totally internally reflected less so less energy loss
  • signals straighter and shorter so faster transmission
67
Q

Order of electromagnetic wave spectrum

A

real monkeys insist very useful xmas gifts

radio microwave infrared visible ultraviolet x-ray gamma

68
Q

Describe how to draw refracted ray when refractive index of both mediums is known

A
  • decrease in refractive index means refract away from normal
  • increase in refractive index means refract towards normal
69
Q

Uses of optical fibres

A
  • endoscopes in medical diagnosis

- communication for faster transmission

70
Q

Describe how to draw diagram when incident ray between two mediums equals critical angle

A
  • light ray travels across boundary

- weak internally reflected ray (dashed)

71
Q

Suggest safety measures taken when using lasers

A
  • avoid shining at a person
  • LASER safety goggles
  • avoid reflections

Physics (5 decks)

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