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A Level Physics - UVI B > Waves and Optics > Flashcards

Flashcards in Waves and Optics Deck (63)
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1
Q

Define monochromatic (1)

A

Single wavelength

2
Q

Define coherent (1)

A

Constant phase difference

3
Q

State a safety precaution when using a laser (1)

A

Do not look into laser

4
Q

State and explain the effect of using a laser with a shorter wavelength on the maxima spacing in Young’s double slit (2)

A

Maxima closer together

use w=𝜆D/s

5
Q

Use wave theory to explain how the fringe pattern is formed in Youngs double slit experiment (4)

A
Slits act as coherent sources
Waves diffract at slits
Waves superpose - 
Bright patches: constructive - in phase 
Dark patches: destructive - 180 degrees out of phase
6
Q

State two requirements for two light sources to be coherent (2)

A

Same wavelength

Same phase

7
Q

Explain how Young’s double slit arrangement produces interference fringes, refer to width of first slit and coherence of second slits (6)

A

Narrow single slit gives wide diffraction to ensure that both second slits are illuminated
Paths to second slits are of constant length giving constant phase difference
Light diffracted at slits and overlap and interfere
Where path lengths differ by whole number of wavelengths, constructive interference occurs producing a bright fringe
Where path lengths differ by 0.5 x n wavelengths, destructive interference occurs producing a dark fringe

8
Q

If Young’s double slit was carried out with red light and then with white light, how would the two differ? (3)

A

Central fringe would be white
Dark fringes would be narrower together
Side fringes are spectra

9
Q

State two ways a diffraction pattern on single slit would change if the slit became narrower (2)

A

Increased separation - see single slit equation

Lower intensity - less light could get through

10
Q

State and explain what happens to angle θ in λ = d sin θ when wavelength decreases? (2)

A

Angle θ gets smaller

As path difference gets smaller

11
Q

Why will total internal reflection occur when light ray travels from water into glass? (refractive index) (1)

A

TIR only occurs when ray travels from higher n to lower n

as long as the incident angle is greater than the critical angle

12
Q

What is the name for the part of an optical fibre that is around the core? (1)

A

Cladding

13
Q

State and explain an advantage of a smaller diameter core (2)

A

Reduce multipath dispersion

Which would cause poor resolution

14
Q

State one application of optical fibres and its benefit to society (2)

A

Communications

Improved transmission of data

15
Q

Explain why optical fibres used for communication need to have cladding (2)

A

Keeps signal secure

Keeps most light rays in

16
Q

State and explain two physical properties of the light produced by a laser which makes it different to that produced by a lamp (4)

A

Monochromatic
Waves of single wavelength
Coherent
Waves produced in constant phase

17
Q

Explain how glass cladding around the optical fibre’s core improves the security of data being transmitted through it and give a reason (3)

A

Light doesn’t enter cladding so can’t pass across from one fibre to a neighbouring fibre
Fibres without cladding can allow light to pass between fibres when scratched or linked by moisture
Personal data must be transmitted along fibres where there is no danger of light leakage

18
Q

Define longitudinal wave (1)

A

A wave with a direction of vibration that is parallel to the direction of propagation of the wave

19
Q

Define transverse wave (1)

A

A wave with a direction of vibration that is perpendicular to the direction of propagation of the wave

20
Q

Explain why it is important to correctly align the aerial of a TV to receive the strongest signal (2)

A

Transmitted radio waves are often polarised

Aerial rods must be aligned in the same plane of the wave

21
Q

Define amplitude of a wave (1)

A

The maximum displacement of the wave from the equilibrium position

22
Q

If there are two polarising filters inbetween an observer and light, if one is turned about 360 degrees, what would the observer see? (2)

A

Variation in intensity between max and min

Two maxima in 360 degree rotation

23
Q

Give three examples of waves that are transverse (2)

A

Electromagnetic radiation
Surface of water
Rope

24
Q

State one application of a polarising filter and a reason for its use (2)

A

Camera/sunglasses

Reduce glare

25
Q

A microwave transmitter directs waves towards a metal plate. A detector is placed between and moved along a line, what causes the maxima and minima of waves detected? (3)

A

Superposition
reflection from metal plate
two waves of same frequency
travelling in opposite directions

26
Q

How can a guitarist raise the fundamental frequency of vibration in their string? (1)

A

Tighten the string
shorter length of string
Use a string of lower mass per unit length

27
Q

Describe the structure of a step-index optical fibre outlining the purpose of the core and cladding (3)

A

Core is transmission medium for electromagnetic waves to progress by total internal reflection
Cladding has a lower refractive index to allow total internal reflection to occur
Cladding offers protection from scratching that could lead to light loss

28
Q

Blue light has a higher refractive index fibre than red light, explain how this difference results in a change in a pulse of white light by the time it leaves the fibre (2)

A

Blue travels slower than red due to greater refractive index

Red reaches the end of the fibre before blue leading to material pulse broadening

29
Q

Discuss two changes to reduce material pulse broadening of white light(2)

A

Monochromatic source so speed is constant

Shorter distance between repeaters so pulse is reformed before significant broadening occurs

30
Q

Suggest an experiment to demonstrate the wave nature of sound (1)

A

Diffraction through a door

31
Q

Which quantities are changed when monochromatic light passes from air into glass? (1)

A

Speed and wavelength only, frequency remains constant

32
Q

Of the electromagnetic waves, which has the greatest energy? (1)

A

Gamma rays

33
Q

How do we know waves transfer energy?

A

Electromagnetic waves heats things up when they are absorbed
X-rays and gamma rays knock electrons out of their orbits (ionisation)
Loud sounds cause oscillations of air particles which make things vibrate.
Waves power can be used to generate electricity.

34
Q

Define wavelength

A

The distance from a point on the wave to the same point on the adjacent wave. It is measured in meters

35
Q

Define frequency

A

Is the number of waves that pass a point every second. It is measured in hertz (Hz)

36
Q

Define period

A

It is the time taken for each one whole wave to pass a fixed point. It is measured in seconds (s)

37
Q

Define phase difference

A

the amount by which one wave lags behind another wave.

38
Q

What units can phase difference be measured in?

A

Phase and phase difference can be measured in degrees, radians or fractions of a cycle.

39
Q

Describe an experiment to measure the speed of sound

A

Start with signal generator + microphones in phase.
Move microphone and measure the distance for 1 wavelength.
Measure the time delay using the picoscope and collect a range of distance and time measurements.
Draw a graph of distance against time.
Use the gradient to find the speed of sound.

40
Q

Define polarisation

A

Restriction of a wave to a single plane

41
Q

What waves can be polarised?

A

Transverse

42
Q

Define interference

A

Interference – When two waves arrive at the same point and at the same time, the resultant displacement is given by the algebraic sum of the two individual displacements.

43
Q

Define constructive interference

A

If two waves meet exactly in phase the amplitudes add up to produce large crests and troughs

44
Q

Define destructive interference

A

If two waves meet exactly out of phase the amplitudes cancel to produce no crests and troughs

45
Q

State the difference between a progressive wave and a standing wave.

A

Progressive (travelling wave) – transfer energy from one region to another

Standing (stationary waves) – Are fixed in space and do not travel. They are caused by the superposition of two identical waves.

46
Q

How are stationary waves formed?

A

Vibrations cause waves to travel along the string towards the pulley.
The waves reflect from the pulley and travel back to the vibration generator.
The two waves meet and constructively and destructively interfere.

47
Q

What are the conditions required for standing waves to be formed?

A

Same frequency and wavelength. Similar amplitude in the medium. Same speed

48
Q

Define a node

A

NO DisplacEment – Positive displacement from one wave is cancelled by an equal negative displacement from the other wave. (Destructive interference)

49
Q

Define an antinode

A

A point of maximum displacement (constructive interference)

50
Q

For a string that is fixed at both ends what are the rules for nodes and antinodes?

A

There must be a node at each end.

51
Q

State 3 ways to increase the fundamental frequency of a guitar string

A

Shorter string
Higher tension
Lower mass per unit length

52
Q

For a closed pipe what are the rules for nodes and antinodes

A

The closed end of the pipe is always a node, the open end of the pipe is always an antinode

53
Q

For an open pipe what are the rules for nodes and antinodes

A

There must be an antinode at each end

54
Q

Define diffraction

A

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

55
Q

When is diffraction noticeable?

A

When the wavelength is similar in size to the gap/obstacle

56
Q

Using the single slit equation state 2 ways to increase the angle of diffraction

A

Larger wavelength

Smaller gap size

57
Q

Define coherence

A

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

58
Q

Using the double slit equation state 3 ways to increase the width between the fringes

A
  • Larger wavelength
  • Increased distance to screen
  • Decrease the slit separation
59
Q

In fibre optics cables why is absorption a problem?

A

Absorption is where some of the signal’s energy is absorbed by the material of the fibre. This energy loss results in the amplitude of the signal being reduced.

60
Q

In fibre optics cables why is dispersion a problem?

A

dispersion cause pulse broadening. The received pulse is broader than the initial signal. Broadened pulses can overlap each other leading to information loss.

61
Q

What is modal dispersion?

A

Is caused by light rays entering the cable at different angles. This causes them to take different paths and therefore take different amounts of time to travel.

62
Q

How can modal dispersion be reduced?

A

To reduce modal dispersion by using a single mode fibre in which light is only allowed to follow a very narrow path.

63
Q

What is material dispersion

A

Is caused by different amounts of refraction expienced by different wavelengths of light. This will cause some wavelengths to be slowed down more and take longer to travel down the cable.