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A-Level Physics > Physics 3 - Waves > Flashcards

Physics 3 - Waves Flashcards

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1
Q

Define frequency and give its units?

A

The number of waves passing through a point per second, it has the units Hertz (Hz).

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2
Q

Define wavelength.

A

The distance between two adjacent peaks or equivalent points on a wave.

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3
Q

Define 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
Where:
T=Time period
f=Frequency

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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. It is measured in radians, degrees or fractions of a cycle.

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6
Q

What is a longitudinal wave?

A

A wave in which the oscillation of the particles is parallel to the direction of energy transfer. They consist of a series of rarefactions (where the particles are more spread out) and compressions (where the particles are closer together).

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7
Q

What is a transverse wave?

A

Waves where the particle oscillations are perpendicular to the direction of energy transfer.

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8
Q

What is an example of a longitudinal wave?

A

Sound waves.

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9
Q

What is an example of a transverse wave

A

Electromagnetic waves.

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10
Q

How fast do electromagnetic waves travel in a vacuum?

A

3×10⁸m/s

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11
Q

True or false; “The magnetic field and electric field in an electromagnetic wave are parallel to each other.”?

A

False, the electric and magnetic fields are at right angles to each other.

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12
Q

What does a polarising filter do?

A

It only allows oscillations in one plane.

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13
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 (as they are in transverse waves).

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14
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, so the receiving aerial must be aligned in the same plane of polarisation to receive the signal at full strength

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15
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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16
Q

What is a node?

A

A point on a stationary wave where the displacement is 0.

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17
Q

What is an antinode?

A

A point on a stationary wave with maximum displacement.

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18
Q

What are the conditions for a stationary wave to be produced?

A

The waves must be of the same frequency, wavelength and amplitude. They must also be travelling in opposite directions.
These conditions are often met when a wave is reflected back onto itself.

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19
Q

How are stationary waves produced?

A

A stationary wave is formed from the superposition of two progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude.
When waves meet in phase, constructive interference occurs, so antinodes (regions of maximum amplitude) form.
When waves meet completely out of phase (in antiphase), destructive interference occurs and nodes (regions of no displacement) form.

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20
Q

Describe the first harmonic for a stationary wave with two closed ends.

A

It consists of two nodes at either end and an antinode in the middle.

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21
Q

Describe the second harmonic for a stationary wave with one open end and one closed end.

A

It consists of two nodes and two antinodes. with one of the nodes at the closed end and one of the antinodes at the open end.

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22
Q

Define coherence.

A

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

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23
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.

24
Q

What was young’s double-slit experiment?

A

A single light source is directed towards two slits, which act as a coherent light source, the light interferes constructively and destructively to create an interference pattern.

25
Q

Describe the interference pattern created using white light.

A

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

26
Q

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

A

The 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.

27
Q

True or false; “Increasing the slit width of a single slit increases the width of the central diffraction maximum.”?

A

False, the slit is not so close to the wavelength in size so less diffraction occurs.
The central maximum becomes narrower and more intense.

28
Q

What is the approximate refractive index of air?

A

1

29
Q

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

A

Towards the normal.

30
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,

31
Q

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

A

It protects the core from scratches which would allow light to escape and degrade the signal.
It allows total internal reflection as it has a lower refractive index than the core.

32
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.

33
Q

What is pulse broardening?

A

When the received signal is wider than the original, which can cause an overlap of signals, which can lead to a loss of information.

34
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, so the rays take different times to travel along the fibre, causing pulse broadening.

35
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.

36
Q

How can modal dispersion be reduced?

A

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

37
Q

How can material dispersion be reduced?

A

Use monochromatic light.

38
Q

How can both absorption and dispersion be reduced?

A

Use an optical fibre repeater to regenerate the signal now and then.

39
Q

State the advantages of optical fibres over traditional copper wires.

A

The signal can carry more information as light has a high frequency.
No energy is lost as heat.
No electrical interference is experienced.
It is cheaper.
The speed of signal transfer is very fast.

40
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, so the angle of refraction is 90°.

41
Q

What formula can be used to find the critical angle for 2 materials whose refractive indices are known?

A 
sin(C)=n₂/n₁ 
where:
C=Critical angle
n₁=Refractive index of material 1
n₂=Refractive index of material 2
and n₁>n₂
42
Q

What is the equation of Snell’s law of refraction?

A 
n₁sin(i)=n₂sin(r)
Where:
n₁=Refractive index of material 1
n₂=Refractive index of material 2
i=Angle of incidence
r=Angle of refraction
43
Q

What does a higher refractive index suggest?

A

The material is more optically dense, so light travels more slowly through it.

44
Q

What formula is used to determine the refractive index of a material?

A 
n=c/v
Where:
n=Refractive index
c=Speed of light in a vacuum (3×10⁸ms⁻¹)
v=Speed of light in material
45
Q

State 2 applications of diffraction gratings.

A

Splitting up light from stars to make line absorption spectra, which is used to identify elements present in the star.
X-ray crystallography, a crystal sheet acts as the diffraction grating, the X-rays pass through this sheet, which is used to find the spacing between atoms.

46
Q

Derive the formula dsin(θ)=nλ

A

For the first order maximum, the path difference between two adjacent rays of light is 1λ, the angle between the normal to the grating and the light ray is θ.
A right angled triangle is formed with side lengths of d (hypotenuse) and λ (opposite) with the angle θ.
For the first maximum, sin(θ)=λ/d which rearranges to dsin(θ)=λ.
Other maxima occur when the path difference between the two rays of light is nλ, where n is an integer, replace λ with nλ to get dsin(θ)=nλ.

47
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, so the orders will become further apart.

48
Q

What is diffraction?

A

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

49
Q

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

A

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

50
Q

What are four 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.

51
Q

What formula is associated with Young’s double slit experiment?

A 
w=λD/s
Where:
w=Fringes spacing
λ=Wavelength
D=Distance from the screen to the slits
s=Slit separation
52
Q

What is path difference?

A

The difference in distance travelled by 2 waves.

53
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 shake from antinodes and settle at the nodes. The distance between each node is half a wavelength.

54
Q

What is the formula for the first harmonic of a string?

A 
f=(1/2l)√(T/μ)
Where:
f=Frequency of the first harmonic
l=Length of string
T=Tension in string
μ=Mass per unit length of string
55
Q

What is the formula for wave speed?

A 
c=fλ
Where:
c=Wave speed
f=Frequency
λ=Wavelength
56
Q

What is ‘phase’?

A

The position of a certain point on a wave cycle, it can be given in radians, degrees or fractions of a cycle.

57
Q

True or False: “Only light can produce interference patterns.”?

A

False, interference patterns can be formed by sound waves and all EM waves too.

A-Level Physics (9 decks)

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