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Physics > Waves > Flashcards

Waves Flashcards

1
Q

what are progressive waves

A

A wave that transfers energy from one point to another without transferring the medium itself

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2
Q

frequency

A

Number of waves passing a certain point per second/per unit time. Measured in Hertz (Hz) or s-1

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3
Q

Amplitude

A

the maximum displacement of a particle in the wave from its equilibrium position

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4
Q

Wavelength

A

the distance between points on successive oscillations of the wave that are in phase

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5
Q

Displacement

A

the distance of a point on the wave from its equilibrium position. It is a vector quantity; it can be positive or negative

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6
Q

Period

A

the time taken for one complete oscillation or cycle of the wave

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7
Q

frequency =

A

1 / time period

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8
Q

wavespeed =

A

wavelength x frequency

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9
Q

what is phase difference between 2 waves

A

a measure of how much a point or a wave is in front or behind another

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10
Q

where can wave difference be found from

A

the relative position of the crests or troughs of two waves of the same FREQUENCY

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11
Q

what does it mean if a wave is in ‘phase’

A

when the crests or troughs are aligned

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12
Q

what does it mean if a wave is in ‘antiphase’

A

when the crest of one wave aligns with the trough of another

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13
Q

what is phase difference measured as

A

fractions of a cycle/wavelength, degrees or radians

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14
Q

how can the phase difference between 2 points be described as

A

in phase it is 360 degrees or 2 pi radians

in anti-phase it is 180 degrees or pi radians

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15
Q

what are the 2 types of waves

A

longitudinal and transverse

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16
Q

transverse

A

a wave in which the particles oscillate perpendicular to the direction of energy transfer of the wave

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17
Q

longitudinal

A

a wave in which the particles oscillate parallel/in the same direction to the direction of energy transfer and travel of the wave

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18
Q

what does a transverse wave look like

A

it shows areas of crests and troughs

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19
Q

what are examples of transverse waves

A

EM waves such as radio, visible and UV
vibrations on a guitar string, S waves

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20
Q

can transverse waves be polarised

A

yes

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21
Q

do transverse waves need a medium to travel in

A

no

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22
Q

what do longitudinal waves look like

A

they have areas of compressions and rarefactions

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23
Q

what are compressions

A

regions of increased pressure

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24
Q

what are rarefactions

A

regions of decreased pressure

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25
Q

what are examples of longitudinal waves

A

sound waves and ultrasound waves and on a slinky spring, P waves

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26
Q

can longitudinal waves be polarised

A

no

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27
Q

do longitudinal waves need a medium to travel in

A

yes

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28
Q

how is energy transmitted through a longitudinal wave

A

the particles in the medium are vibrating as they are given energy

the compressions cause the nearby particles to also vibrate with more energy

this produces a compression further along in the medium

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29
Q

what is the motion of particles as a transverse wave passes by

A

Up to maximum/crest
-Down to equilibrium position
-Down to minimum position/ trough
-Up to equilibrium position

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30
Q

what is the motion of particles as a longitudinal waves passes by

A

particles moves back and forth

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31
Q

what speed do all EM waves travel at in a vacuum

A

3 x 10^8 m/s

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32
Q

what is the movement of particles in a longitudinal wave

A

vibrate left and right but do not move

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33
Q

what is the movement of particles in a transverse wave

A

up and down

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34
Q

1 wave cycle =

A

360 degrees of 2 pi radians

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35
Q

what is phase difference

A

a measure of how much a point or a wave is in front or behind another.

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36
Q

what can phase difference be measured in

A

degrees, radians or fractions of a wave cycle

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37
Q

how do you know if two waves arent in phase

A

if one wave is ahead or behind the other wave

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38
Q

how do you know if 2 waves are in antiphase

A

if the trough of one wave aligns with the crest if another wave

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39
Q

what is polarisation

A

when particle oscillations occur in only one of the directions perpendicular to the direction of wave propogation

OR

The restriction of a wave so that it can only oscillate in a single plane.

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40
Q

why does polarisation occur only in transverse waves

A

because transverse waves oscillate in any plane perpendicular to the propogation direction

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41
Q

why does polarisation not occur in longitudinal waves

A

the particles in a longitudinal wave always oscillate parallel to the direction of energy transfer therefore you cannot isolate a particular direction of vibration from it

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42
Q

how can you polarise waves

A

through a polariser or polarising filter, as they only allow oscillations in a certain plane to be transmitted

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43
Q

how can light be polarised

A

through reflection, refraction and scattering

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44
Q

how does a polaroid filter work

A

it removes all the different planes of the wave except one

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45
Q

what speed do transverse waves travel at

A

all transverse waves travel at the same speed in a vacuum

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46
Q

what happens when two polarising filters perpendicular to each other are used

A

all the light is blocked out because the two filters filter out waves in different planes so completely block out all the wave.

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47
Q

what happens when two polarising filters parallel to each other are used

A

the first filter will polarise the light in 1 axis/direction

All the polarised light will pass through the 2nd filter unaffected

Here, the maximum light intensity is transmitted

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48
Q

what is the intensity of light transmitted when two filters perpendicular to each other are used

A

minimum intensity of light is transmitted

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49
Q

what are the uses of polarisers

A

polarising sunglasses
TV and radio signals
cameras

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50
Q

what do polarising sunglasses do

A

reduce the glare of reflected light

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51
Q

what happens to light when it is reflected

A

it is partially polarised

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52
Q

why are the filters in polarising sunglasses oriented

A

so they cut out light reflecting from horizontal surfaces such as water, snow and tarmac

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53
Q

why do the polarising sunglasses not allow any horizontal light though

A

the polarising filters have vertically oriented transmission axis

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54
Q

why do TV and radio aerials need to be correctly aligned

A

to get the best reception and signal

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55
Q

how can you reduce interference between nearby transmitters

A

if one of the two transmitters is vertically aligned and other is horizontally aligned

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56
Q

how would a graph showing light intensity at different angles look like

A

at o degrees - max light intensity
at 90 - minimum
at 180 - maximum
at 270 - minimum
at 360 - maximum

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57
Q

describe what an observer would see as the 2nd polarising filter is rotated through 360 degrees (2)

A

they would see a variation in intensity between the max and minimum. There will be two maxima in a 360 degrees rotation

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58
Q

explain why it is important to correctly align the aerial of a TV in order to receive the strongest signal

A

transmitted radio waves are often polarised
Aerial rods must be aligned in the same plane of the wave

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59
Q

when does refraction occur

A

when light passes a boundary between two different transparent mediums/media

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60
Q

why does refraction occur

A

one side of the wavefront crosses the boundary first, changing its speed hence causing the wavefront to change directions

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61
Q

what happens to the light ray when it goes from a MORE DENSE to a LESS DENSE medium

A

the light ray speeds up and bends away from the normal

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62
Q

what happens when the light ray goes from a less dense to more dense medium

A

the light ray slows down and bend towards the normal

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63
Q

what changes during refraction

A

the speed and wavelength but NOT the frequency

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64
Q

what is refractive index

A

a property of a material that measures how much light slows down when passing through it

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65
Q

equation for refractive index

A

speed of light in a vacuum / speed of light in a substance

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66
Q

what does the speed that the light travels at depend on

A

the refractive index of the substance

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67
Q

higher refractive index means that …..

A

it is optically dense

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68
Q

what is the refractive index of air

A

1

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69
Q

why is the refractive index of air 1

A

because light doesn’t slow down significantly when travelling through air

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70
Q

equation for snell’s law

A

n1 sin θ1 = n2 sin θ2

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71
Q

what is n1

A

the refractive index of material 1

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72
Q

what is n2

A

the refractive index of material 2

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73
Q

what is theta 1

A

angle of incidence of the ray in material 1

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74
Q

what is theta 2

A

angle of refraction of the ray in material 2

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75
Q

at what angle of refraction is light reflected across the boundary

A

90 degrees and the angle of incidence here is the critical angle

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76
Q

sin theta c =

A

n2 / n 1 , where theta c is the critical angle

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77
Q

when does TIR occur

A

when the angle of incidence is greater than the critical angle and the incident refractive index, n1, is greater than the refractive index of the material at the boundary n2 (more dense to less dense)

78
Q

what are the conditions for TIR

A

angle of incidence has to be greater than the critical angle

the refractive index n1 is greater than refractive index n2

79
Q

what are fibre optics used to do

A

they use TIR to send high speed light signals over large distances

80
Q

what are the uses of fibre optics

A

communications ( telephone and internet transmission)

Medical imaging ( endoscopes)

81
Q

what are the 3 main components of fibre optics

A

1 . Optically dense core such as glass

  1. Lower optical dense cladding surrounding the core
  2. An outer sheath
82
Q

What happens if n of cladding is greater than n of core

A

TIR cannot occur

83
Q

what is the purpose of the outer sheath

A

prevents physical damage to the fibre
strengthens the fibre
protects the fibre from scratches on outside

84
Q

what is the purpose of cladding

A

protects core from damage
keeps signals secure
prevents scratching of the core
Provides fibre with strength ( preventing breakage)

85
Q

when does material dispersion occur

A

when white light is used instead of monochromatic light

86
Q

why does material dispersion occur when white light is used

A

different wavelengths of light travel at different speeds e.g. blue light is slower than red due to greater refractive index

87
Q

what is pulse broadening

A

the elongation of a signal passed down an optical fibre, commonly due to modal or material dispersion

88
Q

how do you prevent pulse broadening

A

By using monochromatic light source so speed of pulse is constant
Make core narrow as possible to reduce possible differences in path length of the signal
Optic fibre repeaters so pulse is regenerated before significant pulse broadening takes place

89
Q

when does modal dispersion occur

A

occurs when rays inside an optical fibre take slightly different paths. Rays taking longer paths take longer to travel through the fibre, so the duration of the pulse increases and the pulse broadens.

90
Q

why is modal dispersion more common in wider cores

A

the light travelling along the axis of the core travels a shorter distance than light undergoing TIR at the core/cladding boundaries

OR

these fibres are broad enough to allow rays to take different paths

91
Q

how do you prevent MODAL dispersion

A

Use narrow cores

92
Q

What are the advantages of a narrow core

A

less light is lost by refraction

less overlapping pulse so modal dispersion decreases

Quality of signal improves and is less distorted

Signal is transferred quicker

Angle of incidence is less likely to be smaller than the critical angle

93
Q

what is absorption of signal

A

part of the signals energy is absorbed by the fibre and some wavelengths are absorbed and so the signal becomes weaker

94
Q

what does absorption do

A

reduces the amplitude of signal so there is a loss of information and signal strength falls

95
Q

what does pulse broadening cause

A

different pulses to merge which leads to completely distorted final pulse

96
Q

how can you reduce absorption

A

use an extremely transparent core

Use optical fibre repeaters

97
Q

why does optical fibre repeaters reduce absorption

A

so the pulse is regenerated before the signal absorption occurs

98
Q

how do you reduce pulse broadening

A

core should be as narrow as possible

use monochromatic light source

use optical fibre repeaters

99
Q

why does using a narrow core reduce pulse broadening

A

it reduces the possible differences in path length of the signal and light is very nearly confined to one single path along the axis of the cable

100
Q

why does using a monochromatic source reduce pulse broadening

A

the speed of the pulse will be constant

101
Q

What is superposition

A

2 waves of the same type meet at the same point and overlap.

The resultant displacement is the vector sum of the displacements of each wave

102
Q

What is superposition used for

A

Reducing unwanted noise

103
Q

How are stationary / standing waves produced

A

by the superposition of 2 waves with the SAME frequency and amplitude travelling in opposite directions

104
Q

When are stationary waves often produced

A

when reflections of a progressive wave superpose with original wave

105
Q

what happens in constructive superposition

A

the 2 displacements add up

106
Q

what happens in deconstructive superposition

A

the 2 displacements cancel out

107
Q

when does constructive superposition occur

A

when the 2 waves are in phase

108
Q

when does deconstructive superposition occur

A

when the 2 waves are out of phase

109
Q

what are the differences in amplitude between progressive and stationary waves

A

progressive - all points (in turn) have same amplitude

stationary - each point has different amplitude depending on amount of superposition

110
Q

what are the differences in phase between progressive and stationary waves

A

progressive - points exactly a wavelength apart are in phase

stationary - points between nodes are in phase. Points either side of a node are out of phase

111
Q

what are the differences in energy between progressive and stationary waves

A

progressive - energy is transferred

stationary - energy is stored

112
Q

what are the differences in speed between progressive and stationary waves

A

progressive - speed is the speed through the medium

stationary - each point oscillates as a different speed. The overall wave doesnt move

113
Q

Nodes

A

Points with 0 displacement on a stationary wave. They do not move at all

114
Q

Antinodes

A

Points of maximum displacement on a stationary wave. They only move vertically

115
Q

Why do particles not oscillate at a node

A

The amplitudes of the 2 waves moving in opposite directions always cancel out so particles dont oscillate. Destructive interference

116
Q

What are the points of deconstructive interference called

A

nodes

117
Q

what are the points of constructive interference called

A

antinodes

118
Q

How much are 2 nodes / 2 antinodes seperated by

A

half a wavelength

119
Q

conditions for superposition

A

waves must be travelling in opposite directions

same frequency , amplitude and wavelength

120
Q

How is sound in guitars for example, produced

A

vibrations caused by stationary waves produce sound

121
Q

How are stationary waves produced by microwaves

A

Place a microwave source in line with a reflector with a detector in the middle

Reflector can be moved to vary the stationary wave pattern

When you move the detector, it picks up the nodes and antinodes

122
Q

How are sound produced in instruments such as organs

A

Produced as a result of the formation of sound waves inside an air column

123
Q

How are stationary waves produced by sound waves

A

Place fine powder in an air column and a loudspeaker at the open end

At certain frequencies, the powder is evenly spread to show places of 0 disturbance due to the nodes

There must be a node at one end and an antinode at the end with the loudspeaker, to produce the stationary wave

124
Q

Harmonics

A

Different modes of vibration

125
Q

1st Harmonic

A

Mode of vibration with the longest wavelength

126
Q

2nd Harmonic

A

Mode of vibration with the 2nd longest wavelength

127
Q

frequency of 1st harmonic =

A

1/2 length of string x root (tension in string / mass per unit length of string )

128
Q

speed of wave travelling across 2 fixed ends =

A

root ( tension in string / mass per unit length of string )

129
Q

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

A

Core is a transmission medium for EM waves to progress by total internal reflection

Cladding provides lower refractive index so total internal reflection can occur

Cladding also offers protection of boundary from scratching which could lead to light leaving the core

130
Q

How to reduce effect of pulse broadening caused by material dispersion (2)

A

Use a monochromatic light source so the speed of the pulse is constant

Use optic fibre repeaters to regenerate the signal before significant pulse broadening occurs

131
Q

what happens inside microwaves when cooking food

A

Microwaves are generated by a magnetron

The waves reflect off the metallic inner surfaces to ensure the food cooks evenly

132
Q

What happens to the food at the nodes and antinodes in a microwave

A

nodes - undercooked

Antinodes - overcooked

This is why microwaves have rotating turntables

133
Q

Explain how a receiver, vertical metal rod and transmitter are used to demonstrate that the waves from the transmitter are vertically polarised (3)

A
  1. Rotate the aerial in the vertical plane
  2. When the aerial is vertical, the signal is at max
  3. When the aerial is horizontal, the signal is at min
134
Q

Explain what happens to the signal detected by the receiver as the metal plate is moved slowly towards a point X (4)

A
  1. Received signal goes through max and min
  2. Reflected and direct microwaves interfere
  3. Path difference increases as the plate is moved
  4. Max is when aerial is aligned with the plane of polarisation of the microwave
135
Q

What properties must 2 waves have to form superposed waves

A

Same frequency and amplitude and speed
Must be travelling in opposite directions

136
Q

how many wavelengths is 2L

A

1

137
Q

Area of a string =

A

pi x diameter^2 / 4

138
Q

mass per unit length of a string using density =

A

pi x diameter^2 x density / 4

OR :

Pi x radius^2 x density

139
Q

What is the aim of the stationary waves RP

A

To measure how the frequency of 1st harmonic is affected by changing :

Length of string
Tension
Different masses per unit length

140
Q

What is the independant variable in the stationary waves RP

A

Length/tension/mass per unit length

141
Q

What is dependant variable in the stationary waves RP

A

Frequency of the 1st harmonic

142
Q

What is the control variable for the stationary RP if the length is varied

A

Same masses attached (tension) and same string (mass per unit length)

143
Q

What is the control variable for the stationary waves RP if the tension is varied

A

Same length of the string and same string(mass per unit length)

144
Q

What is the control variable for the stationary waves RP if the mass per unit length is varied

A

Same masses attached (tension) and same length of string

145
Q

Method for the stationary waves RP

A

Attached 1 end of the string to vibration generator and pass the other end over the pulley and attached mass hangar.

Adjust the position of the bridge so L is measured from vibration generator to the bridge using a meter rules

Turn on the signal generator to set the string oscillating

Increase the frequency of the v. generator till 1st harmonic observed and read at which frequency this occurs

Repeat using different lengths

Repeat frequency readings 3 times and take an average

Measure tension in the string using T = mg

Calculate mass per unit length

Draw a table showing L, F1-3 and mean F

Draw a graph of F against 1/L

146
Q

what is the y axis on the stationary waves Rp

A

frequency

147
Q

What is the x axis on the stationary waves RP

A

1/L

148
Q

What is the gradient on the stationary waves RP

A

V/2

149
Q

What are the systematic errors in the stationary waves RP

A

Use an oscilloscope to verify signal gen readings

Leave signal generator for 20 mins to stabilise

Use as large as possible length to get greater resolution

150
Q

What are the random errors in the stationary waves RP

A

The sharpness of resonance when the 1st harmonic is achieved

151
Q

how to avoid the random error of the sharpness of resonance

A

Dont look at the amplitude as the wave is too fast

Adjust the frequency whilst looking closely at a node

152
Q

Safety procedures in stationary waves RP

A

Use rubber strings instead of metal in case it snaps under tension

Wear goggles to protect eyes

Stand away from masses in case they fall

153
Q

Coherence

A

If 2 waves have the same wavelength and frequency and there is a constant phase difference between the 2 waves

154
Q

When does interference occur

A

When waves overlap and the resultant displacement is the sum of displacements of each wave

155
Q

What is path difference measured in

A

Multiples of wavelength

156
Q

Path difference

A

difference in distance travelled by 2 waves from their sources to the point where they meet

157
Q

What happens to sound when coherent waves are in phase

A

Sound is louder due to constructive interference

158
Q

What is the path difference for constructive interference

A

Path difference of n wavelengths

159
Q

What is the path difference for deconstructive interference

A

Path difference of (n + 0.5)wavelengths

160
Q

What is the safety issues with lasers

A

Lasers produce very high energy beam of light which can cause blindness or permanent eye damage

161
Q

Safety precautions with lasers

A

never look directly at a laser or its reflections

Do not shine the laser towards a person

Wear safety goggles

Stand behind the laser

162
Q

What are the conditions for light for two source interference

A

Wave sources must be coherent and monochromatic

163
Q

Describe what happens when monochromatic lights is shined through a single and double slit

A

Monochromatic light is shone behind a single slit.

Light is diffracted producing 2 light sources at the double slit

Light waves are coherent so create an observable interference pattern

Interference patter is made up of light and dark fringes

164
Q

fringe width =

A

wavelength x distance from double slit to screen / distance between centres of slits

165
Q

What happens at bright fringes

A

constructive interference

166
Q

Why is the distance between fringes very small

A

Due to the short wavelength of visible light

167
Q

What happens to the interference pattern when monochromatic light is replaced with white light

A

Different interference patter forms.

White lights has all colours of visible light.

Each wavelength of v. light produces its own interference pattern

Central fringe is white because the path difference for all the wavelengths is 0 so they are in phase

the 1st. maximum is when the phase difference is 1 wavelength

Each colour produces maximum in different positions so colours spread into sprectrum

168
Q

What was the 1st significant discovery

A

Newton - Visible light is a stream of microscopic particles (corpuscles) but it couldn’t explain diffraction or refraction

169
Q

What was the 2nd significant discovery

A

Huygens - Original wave theory of light (its a series of wavefronts) to explain diffraction and refraction

170
Q

What was the 3rd significant discovery

A

Young - light is a wave that undergoes (de)constructive interference

171
Q

What was the 4th significant discovery

A

James Maxwell - E + M fields obey wave equation and light is made of e + m fields travelling perpendicular

172
Q

What was the 5th and last significant discovery

A

Light behaves as a particle.
Light is described in terms of photons
Light behaves as a wave and particle

173
Q

What is the aim of the Young’s double slit RP

A

To determine the wavelength of light using Young’s double slit apparatus

174
Q

Describe the method for Young’s double slit RP

A

Place the laser in a clamp a few centimeters from the double slit in it’s holder. Place the screen approximately 1 metre from the double slit.

Adjust the position of the laser until an interference pattern is visible on the screen.

Increase the distance ‘D’ between the double slit and the screen to increase the fringe spacing ‘w’. This makes the fringe spacing easier to measure but if D too large the fringes will not be clearly defined.

The fringe width (or fringe spacing), w, can be measured by measuring across a large number of visible fringes. (Take care when counting – counting from the first bright fringe to the tenth bright fringe would represent nine fringe widths!).

Measure across 5 fringes and record the value 5w in your table.

Use the tape measure to measure D and obtain 5 further values for D with the corresponding value of 5w.

Plot a graph of w against D, draw a line of best fit and calculate the gradient. Given that the gradient is
λ/s provide a value for the wavelength of the laser light.

175
Q

What is the independant variable in the Young’s double slit RP

A

Distance between double slit and the screen

176
Q

What is the the dependant variable in the Young’s double slit RP

A

The distance between maxima

177
Q

What are the control variables in the Young’s double slit RP

A

Slit seperation, distance between laser and screen, wavelength of the laser

178
Q

Diffraction

A

Spreading out of waves when they pass an obstruction (usually a narrow slit/arpeture)

179
Q

What does the extent of diffraction depend on

A

High wavelength = more diffraction
Smaller gap = more diffraction

Diffraction is most prominent when the width of the slit is = to the wavelength of the waves

180
Q

What is the only property of a wave which changes when a wave is diffracted

A

Its amplitude because some energy is dissipated when a wave is diffracted through a gap

NOT WAVELENGTH OR WAVESPEED OR FREQUENCY

181
Q

Features of the single slit separation pattern

A

Central maximum with high intensity

Subsidiary maxima equally spaced, successively smaller in intensity and half the width of the central maximum

182
Q

What would happen if the laser was replaced with a non-laser source emitting white light

A

Central maximum would be white

Shortest wavelength would appear nearest to the central maximum

Longest wavelength would appear furthest from central maximum

Fringe spacing smaller and maxima wider

183
Q

What happens if the blue laser was replaced with a red laser

A

Wavelength of red light is longer so light diffracts more
Intensity fringes wider

184
Q

What happens to the intensity and fringe spacing if the slit was made narrower

A

Intensity = decreases as less light goes through
Fringe spacing = Wider

185
Q

equation for angles at which maxima form

A

spacing between adjacent slits x sin0 = order of maxima x wavelength of source

186
Q

Spacing between adjacent slits =

A

1 / number of lines per metre on grating

187
Q

number of maxima =

A

d / wavelength

188
Q

path difference at 0th order

A

0

189
Q

path difference at 1st order

A

1 wavelength therefore there is constructive interference

190
Q

equation for the first order maxima

A

sin0 = wavelength of light x slit separation

191
Q

Uses of diffraction gratings

A

Separating lights of different wavelengths with high resolution
Used in spectrometers
X-ray crystallography

192
Q

How are diffraction grating used in spectrometers

A

To analyse light from stars
Chemical analysis
Measure red shift
Measure frequency or wavelength of light from a star

Physics (10 decks)

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