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A level Physics > Unit 2.5 - The Properties of Waves > Flashcards

Unit 2.5 - The Properties of Waves Flashcards

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1
Q

Diffraction

A

The effect observed when waves pass an object…
When waves strike an object, part of the wave is slowed down, and so the wave front curves
When a wave strikes a space in a barrier, both sides curve

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2
Q

Which waves undergo diffraction?

A

ALL waves

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3
Q

What’s the word for the effect observed when waves pass an object?

A

Diffraction

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4
Q

What happens when waves strike an object?

A

Part of the wave is slowed down, and so the wave front curves

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5
Q

What happens wen a wave strikes a space in a barrier?

A

Both sides curve

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6
Q

How can we show the diffraction of waves in water?

A

Ripple tank

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7
Q

Draw and label a ripple tank

A

(See notes)

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8
Q

Draw a wavefront and label its wavelength

A

(See notes)

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9
Q

What’s the angle between the direction of a wave and the wavefront?

A

90 degree angle

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10
Q

Describe the waves from a narrow gap between barriers in a ripple tank

A

A lot of spread
Waves become more and more curved

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11
Q

Describe the waves from a wide gap between barriers in a ripple tank

A

A little spread
Waves flat in the middle and curve on sides

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12
Q

How come you can hear sounds around corners?

A

A doorway is a good diffraction of sound waves, and sound waves can bend

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13
Q

What type of waves bend the most?

A

The ones with the longest wavelengths

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14
Q

What do waves with a longer wavelength do more?

A

Bend more

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15
Q

What types of sounds are loudest and why?

A

The ones with the longest wavelengths
Curves more

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16
Q

What does the wavelength of a sound wave depend on?

A

The frequency

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17
Q

What does a longer wavelength mean in a sound wave and why?

A

It’s louder as the wave is curved more

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18
Q

What is the wavelength of sound usually, and what is this similar to?

A

1m, similar to the doorway in a house

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19
Q

Why is it not possible to see a light around corners in the same as as we can hear sound around corners?

A

The wavelength of light is a lot shorter - the waves bend less

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20
Q

In which situation would you get the most diffraction?

A

If the gap or the size of the barrier is roughly the same as the wavelength of the diffracting waves

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21
Q

What does a ‘lot of diffraction’ basically mean?

A

A lot of spread of the wave

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22
Q

What do you get if the gap or size of the barrier in the pass of a wave is roughly the same as the wavelength of the diffracting waves?

A

A lot of diffraction

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23
Q

Draw a diagram to explain how the diffraction of microwaves would be shown

A

(See notes)

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24
Q

What do waves do when they interact?

A

“Pass through” each other

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25
Q

What happens in the region where waves overlap?

A

They superpose

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26
Q

What does it mean that waves superpose?

A

They “add up”

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27
Q

Principle of superposition

A

When two waves interact, their combined displacement at any point is the vector sum of the individual waves at that point

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28
Q

What does the ‘vector sum’ part of the principle of superposition show us?

A

The directions of the individual displacements need to be taken into account

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29
Q

Who was the first to demonstrate interference effects using light and when?

A

Thomas Young in the 19th century

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30
Q

What was Thomas Young the first to do?

A

Demonstrate interference effects using light

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31
Q

What first suggested wave-light properties of light?

A

Interference effects

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32
Q

What did interference effects suggest in Young’s experiment?

A

Wave-like properties associated with light

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33
Q

Where the ideas of Thomas Young’s experiment immediately accepted?

A

No, not until much later on

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34
Q

Draw the layout of Thomas Young’s experiment

A

(See notes)

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35
Q

Draw how Thomas Young’s experiment can be adapted to using sound

A

(See notes)

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36
Q

How would the sound version of Thomas Young’s experiment wok?

A

As a person walked from A to B, they would notice that the sound intensity (volume) wold rise and fall as the constructive interferences alternate

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37
Q

Why would the volume rise and fall whilst walked in a straight line by loudspeakers?

A

The constructive and destructive interferences alternate

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38
Q

What do we need to think about when considering interference?

A

The phase of the waves arriving at the screen

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39
Q

What is interference caused by?

A

Having 2 parallel slits for the wave to pass through

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40
Q

What are bright fringes in interference caused by?

A

Constructive interference

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41
Q

Constructive interference

A

The waves superimpose in phase, so the difference in distance both waves have travelled must be a whole number of wavelengths

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42
Q

Describe the interference of waves if they meet in phase

A

Constructive

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43
Q

How must waves meet for it to be constructive interference?

A

In phase

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44
Q

Which part of a wave overlaps with what for constructive interference?

A

Crest with crest

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45
Q

What are dark fringes in interference caused by?

A

Destructive interference

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46
Q

Destructive interference

A

Thaw waves must have travelled a distance that is different by a number of half wavelengths

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47
Q

Which part of a wave interacts with what for destructive interference?

A

Crest with trough

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48
Q

Is it still constructive interference if a point is 4 wavelengths apart from one slit and 3 from the other?

A

Yes, because they’re still in phase as its a WHOLE wavelength

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49
Q

Waves travelling by a distance hat is different by a number of half wavelengths

A

Destructive interference

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50
Q

Difference both waves have travelled is a whole number of wavelengths

A

Constructive interference

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51
Q

What does constructive interference cause?

A

Bright fringes

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52
Q

What does constructive interference cause?

A

Dark fringes

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53
Q

Do wavelengths change during diffraction?

A

No

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54
Q

What should you NOT show with the wave that comes out of a gap in an exam question?

A

Gaps between the ends of the waves and the barrier

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55
Q

How does the frequency of resultant waves from superimposing compare to the incoming waves?

A

The same

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56
Q

Which two conditions must be true for a fringe pattern to be observed with waves passing through slits?

A

The two sources of waves must have a constant phase difference
The vibrations must be in the same line (parallel)

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57
Q

Phase difference

A

The difference in time between 2 similar events

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58
Q

Path difference

A

The difference in distance travelled by 2 similar waves

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59
Q

Coherent

A

Two waves are coherent if thy have a constant phase difference

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60
Q

Give an example of a light which is coherent

A

Laser light

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61
Q

Give an example of a light that ISN’T coherent

A

Light from a filament lamp

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62
Q

What’s the phase difference is the path difference is zero?

A

Zero

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63
Q

What’s the phase difference if the path difference is half a wavelength?

A

Pi

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64
Q

Definition of interference

A

The superposition of two (or more) coherent waves. When this happens, it causes reinforcement at some points and cancellation at other points.
The resulting effect is called a “system of fringes” or an “interference pattern”

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65
Q

What was the first experiment to show the wave nature of light and as opposed to what?

A

Young’s double slit experiment
As opposed to particle nature

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66
Q

What did Young’s double slit experiment show us?

A

That light has a wave nature, not particle nature

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67
Q

Who first completed the double slit experiment and when?

A

Thomas Young
1801

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68
Q

Draw how the double slit experiment is completed

A

(See notes)

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69
Q

What can be used as the light source in the double slit experiment?

A

Either…
Monochromatic light source and a single slit
A laser source

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70
Q

In which direction do waves diffract when maximum diffraction occurs and from where?

A

90 degrees from the ‘straight through’ direction

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71
Q

What would change about the diffraction of a wave if the frequency increased?

A

Wavelength decreases (becomes smaller than the gap)
Less spreading
Side beams

72
Q

When do you get the most diffraction from a wave travelling through a slit?

A

If the wavelength is equal to or greater than the width of the slit

73
Q

What must wave sources have for two source interference to be observed clearly?

A

Zero/Constant phase difference (coherent)
The same frequency
The same or similar amplitudes
Oscillations in the same direction
The same wavelength
2 slits

74
Q

Who believed that waves have particle nature around the time of Young’s experiment?

A

Newton

75
Q

Path difference

A

The difference in the distance travelled by 2 waves in arriving at a particular point on the screen

76
Q

What is the maximum amplitude of a wave caused by?

A

Constructive interference

77
Q

What is the minimum amplitude of a wave caused by?

A

Destructive interference

78
Q

What does constructive interference show up as on the interference pattern?

A

A light fringe

79
Q

What does maximum amplitude of a wave show up as on the interference pattern?

A

Light fringe

80
Q

What does destructive interference show up as on the interference pattern?

A

Dark fringe

81
Q

What does minimum amplitude/zero amplitude show up as on the interference pattern of a wave?

A

A dark fringe

82
Q

What is the path difference in waves that undergo constructive interference?

A

A whole number of wavelengths

83
Q

How do waves arrive if their path difference is a whole number of wavelength?

A

In phase

84
Q

What is the path difference of waves that undergo destructive interference?

A

An odd number of wavelengths

85
Q

How do waves arrive that have a path difference that’s an odd number of wavelengths?

A

In anti phase

86
Q

What does “a” represent in the equation for Young’s experiment?

A

Slit separation

87
Q

What does “y” represent in the equation for Youngs experiment?

A

Fringe separation

88
Q

What does “D” represent in the equation for Young’s experiment?

A

Distance from the double slits to the screen

89
Q

What do we count on a diagram to work out fringe separation?

A

The gaps between bright fringes, not the bright fringes themselves

90
Q

Diffraction grating

A

A piece of glass with a large number of parallel and equidistant lines scratched on it

91
Q

What do the gratings of a diffraction grating do to light?

A

Split it up into its constituent colours

92
Q

What is white light diffracted according to with a diffraction grating?

A

Wavelength

93
Q

What happens when white light is passed through a diffraction grating?

A

Light is “split up” into its constituent colours, and diffracted according to wavelength
So, all the colours of the spectrum can be observed

94
Q

How can we observe all of the colours of the spectrum using white light?

A

Diffract it through a diffraction grating, where its diffracted according to wavelength

95
Q

When will diffraction occur on a diffraction grating?

A

When light is reflected onto the grating or if its projected through the grating

96
Q

What effect does wavelength have on the amount of diffraction of light?

A

Longer wavelength = greater diffraction

97
Q

Which light is deviated most - either or blue light - using the same diffraction grating and why?

A

Red light gets deviated more
Longer wavelength = greater diffraction

98
Q

Draw and label a diagram of white light hitting a grating to diffract light

A

(See notes)

99
Q

What can be observed when using a grating with more lines per mm (reducing d) to diffract light?

A

∆y is larger = beams are further apart (due to more constructive interference)
Bright fringes are sharper = bright maxima are very arrow = the angle can be measured accurately

100
Q

Under which conditions is ∆y larger for diffracted light and why?

A

When using a grating with more lines per mm (reducing d)
More destructive interference

101
Q

What is using a grating with more lines per mm reducing?

A

d

102
Q

Under which conditions are the bright fringes sharper through a diffraction grating and why is this advantageous?

A

Using a grating with more lines per mm (reducing d)
The angle can be measured accurately

103
Q

What does it mean if bright fringes are sharper after passing them through a grating with a lower value for d?

A

Bright maxima are very narrow

104
Q

When are diffraction orders produced?

A

When monochromatic light is incident normally on a diffraction grating

105
Q

What are diffraction orders?

A

A pattern of narrow, bright fringes

106
Q

Monochromatic

A

Single wavelength

107
Q

Normally

A

At right angles

108
Q

Draw and label a diagram to represent diffraction orders

A

(See notes)

109
Q

What do we label the different diffraction orders produced by a diffraction grating?

A

(X)th order maximum

110
Q

What do we call the beam of light that travels in a straight line through the diffraction grating?

A

Straight through beam

111
Q

What are diffraction orders caused by?

A

Constructive interference from light waves arriving in phase

112
Q

How much light waves arrive for constructive interference to occur?

A

In phase

113
Q

What number of maxima is there always for diffraction orders and why?

A

An odd number
Middle maximum, and an equal amount on both sides

114
Q

What do wavelets do as they’re circular?

A

Overlap

115
Q

Why would waves be circular?

A

By the diffraction process

116
Q

What does nearly every part of every wavelet do to their wavelets when passed through 2 slits?

A

Nearly every part of every wavelet will overlap with another wavelet that is in anti phase, causing destructive interference

117
Q

What causes destructive interference?

A

Waves overlapping in antiphase

118
Q

What’s the only direction in which constructive interference can occur?

A

The direction that the pass difference from each slit is a whole number of wavelengths

119
Q

What’s the distance between slits for the first order maximum?

A

A single wavelength

120
Q

What unit to we work in using the diffraction grating equation?

A

Metres

121
Q

Derive the diffraction grating equation

A

(Lovely triangle drawing drawn I’m sure)
d = slit separation
BC = path difference
θ = angle to bright spot

Sin θ = BC
——
AB

path difference
———————-
slit separation (d)

For constructive interference, path difference is nλ (a whole number of wavelengths)

Sin n λ
——
d

dsin(θ) = n λ

122
Q

What’s the diffraction grating equation?

A

dsin(θ) = n λ

123
Q

What is d in dsin(θ) = n λ?

A

Slit separation

124
Q

Symbol and unit for slit separation

A

d
Metres

125
Q

What’s the method for finding the total number of maxima? (diffraction orders)

A

The limit of θ = 90 degrees
So, there’s a limit to the number of orders that can be observed (has a maximum)
n λ = dsinθ
Sin 90 = 1

n= d
—
λ

nmax ≤ d
—
λ

126
Q

Why is there a limit to the diffraction orders that can be observed?

A

The limit of θ = 90 degrees

127
Q

What is sin 90 equal to?

A

1

128
Q

Equation for finding the total number of maximum diffraction orders

A

nmax ≤ d
—
λ

129
Q

What do we NOT do to the value of the maximum order for diffraction orders and why?

A

Round the answer up
It would surpass 90 degrees

130
Q

How do we work out the total maxima if we have the maximum order?

A

Think - 1 on the middle, the value for maximum order on both sides

131
Q

How do we calculate d for the diffraction grating equation?

A

1/lines per mm

Answer divided by 1000 to be in metres

132
Q

How would we re-arrange the diffraction grating equation to find theta for example?

A

Use SHIFT sin

133
Q

coherent light

A

waves or wave sources have a constant phase difference between them and therefore must have the same frequency

134
Q

examples of coherent wave sources

A

lasers, speakers emitting the same frequency sound wave, two dippers on a beam in a ripple tank

135
Q

incoherent light

A

emit waves that have no phase relationship

136
Q

examples of incoherent wave sources

A

lightbulb, LED, person splashing wildly in water

137
Q

what must it follow that a coherent light source is?

A

monochromatic

138
Q

monochromatic

A

single wavelength

139
Q

stationary waves

A

caused by the superposition of identical progressive waves that are travelling in opposite directions

140
Q

what do we mean by ‘identical waves’ in the definition for stationary waves?

A

same amplitude and period

141
Q

how can stationary waves be produced?

A

by reflecting progressive waves from a source back towards the source

142
Q

nodes

A

points of zero amplitude on a stationary wave

143
Q

antinodes

A

points of maximum amplitude on a stationary wave

144
Q

what are nodes caused by in a stationary waves?

A

destructive interference

145
Q

what are antinodes caused by in stationary waves?

A

contructive interference

146
Q

draw and label the nodes and antinodes on a stationary wave

A

(see notes)

147
Q

what’s the distance between the nodes (or antinodes) in a stationary wave?

A

half a wavelength

148
Q

what results in the stationary wave being produced when reflecting a progressive wave from a source back to the source?

A

the interference between the incident wave and the reflective wave

149
Q

what’s the relationship between the wavelength of a stationary wave and the progressive wave that causes it?

A

the same

150
Q

what does a stationary wave have the same wavelength as?

A

the progressive wave that caused it

151
Q

compare progressive and stationary waves in terms of energy

A

progressive - energy is transferred at the wave-front
stationary - energy is captive (i.e - not transferred)

152
Q

compare progressive and stationary waves in terms of amplitude

A

progressive - every particle along the path is vibrated to the amplitude
stationary - varies from zero at the nodes to a maximum at the antinodes

153
Q

compare progressive and stationary waves in terms of frequency

A

progressive - all particles vibrate with the frequency of the wave
stationary - all particles vibrate with the frequency of the wave except those at the nodes (which don’t oscillate at all)

154
Q

what do some of the particles in only a stationary wave have at all times?

A

a displacement of zero

155
Q

compare progressive and stationary waves in terms of wavelength

A

progressive - the distance between adjacent particles having the same phase
stationary - twice the distance between adjacent nodes or adjacent antinodes

156
Q

compare progressive and stationary waves in terms of phase

A

progressive - all particles within one wavelength have a different phase
stationary - all particles between 2 adjacent nodes have the same phase but are in antiphase with those in the next loop

157
Q

Equation for the speed of sound

A

c = f λ

158
Q

What’s c = fλ the equation for?

A

The speed of sound

159
Q

Why does every particle between two adjacent nodes in a stationary wave have the same phase?

A

The wave moves up and down together

160
Q

Fundamental stationary wave

A

The lowest frequency stationary wave

161
Q

What does a fundamental stationary wave look like?

A

1 loop

162
Q

Wavelength of a fundamental stationary wave

A

2L
(Think - node to node is half a wavelength, and this type of wave literally just has 2 nodes - 1 on each end)

163
Q

Harmonic

A

The amount of “loops” in a stationary wave

164
Q

What would a stationary wave with 2 “loops” be known as?

A

The 2nd harmonic

165
Q

Harmonic of a fundamental stationary wave

A

1st harmonic

166
Q

In the equation V^2 = T x L
——
m

If T is reduced by a factor of 4 (i.e - a quarter of the previous value), what factor does V change by?

A

1/4 of its value
√1/4
1/2
Decreases by a factor of 2

167
Q

What do we mean when we say that waves diffract?

A

Spread out

168
Q

What’s the same between 2 coherent waves?

A

Frequency

169
Q

Under which situation would we get lots of diffraction orders?

A

If the skit separation is large compared to the wavelength

170
Q

What’s the difference between the interference pattern with Young’s experiment and using a diffraction grating? Explain

A

(Using a diffraction grating…)
Beams are further apart and there’s fewer of them (more slits and so more destructive interference)
Bright fringes are sharper as the bright maxima are very narrow

171
Q

What happens to the intensity of light a the distance from the “straight through” of diffracted light increases? Why?

A

The intensity decreases
Most intensity “straight through”

172
Q

What will happen to fringe separation as wavelength increases? Why?

A

Increasing wavelength will increase fringe separation
(Directly proportional)

173
Q

How can we increase fringe separation?

A

Increase distance from screen

174
Q

Why must constructive interference occur in deriving the diffraction grating equation?

A

For waves to arrive in phase to cause a bright spot

175
Q

Which word should we always use when describing a stationary wave?

A

Reflecting

176
Q

Do path differences need to be inserted into the equation?

A

Yes because it IS the path difference equation

A level Physics (29 decks)

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