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Physics The Good One > 05 waves and the nature of light > Flashcards

05 waves and the nature of light Flashcards

1
Q

longitudinal waves

A

a type of wave in which the particles oscillate parallel to the direction of the wave

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2
Q

wavelength

A

the distance between two matching points in neighbouring waves

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3
Q

amplitude

A

the maximum displacement a point moves from its centre of oscillation

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4
Q

The larger the amplitude…

A

the greater the energy

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5
Q

period

A

the time taken for a point on a wave to move through one complete oscillation

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6
Q

frequency

A

the number of oscillations per second

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7
Q

frequency =

A

1/time period

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8
Q

describe electromagnetic waves

A

transverse waves made up of electric and magnetic fields which transfer energy from one place to another. All of the waves travel the same speed in a vacuum.

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9
Q

higher frequency means…

A

more energy

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10
Q

what is the order of the electromagnetic spectrum?

A

radio

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11
Q

what is the wavelength of a radio wave?

A

10^3 - 10^1 m

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12
Q

what is the wavelength of a microwave?

A

10^-2 m

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13
Q

what is the wavelength of a Infra-red wave?

A

10^-5 m

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14
Q

what is the wavelength of a visible light wave?

A

10^-7 m

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15
Q

what is the wavelength of a ultra violet wave?

A

10^-8 m

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16
Q

what is the wavelength of a x-ray wave?

A

10^-10 m

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17
Q

what is the wavelength of a gamma ray?

A

10^-12 m (+)

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18
Q

diffraction

A

is the spreading out of a wave as it goes past an obstacle or through a gap

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19
Q

What is Huygens principle?

A

a model where each point on a wave front may be regarded as a source of waves expanding from that point. It allowed a visualisation of how light could penetrate into geometric shadow in a way that particles could not

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20
Q

What is a diffraction grating?

A

an optical component with a periodic structure that splits and diffracts light into several beams travelling in different directions.

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21
Q

constructive interference

A

is known as in phase

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22
Q

What is superposition?

A

when the wave amplitudes are added

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23
Q

destructive interference

A

is known as out of phase

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24
Q

degrees to radians conversion

A

radians = (degrees * pi)/180

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25
Q

radians to degrees conversion

A

degrees = (radians * 180) / pi

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26
Q

For two waves of light to be coherent the waves must

A

originate from one source

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27
Q

how does an additional converging lens affect the eye

A

decreases the image distance as the lens adds more power

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28
Q

virtual image

A

cannot be projected onto a screen

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29
Q

unit of viscosity

A

PaS

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30
Q

why does intensity decrease over distance

A

the area the wave is spread out over is larger so the intensity is lower (interference can also affect the intensity)

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31
Q

wave speed

A

the speed the wave travels. v = fλ

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32
Q

wave equation

A

v = fλ

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33
Q

wave

A

something that transfers energy from one point to another

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34
Q

describe a longitudinal wave

A

particles in metal vibrate along direction of propagation making compressions and rarefactions

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35
Q

frequency time period equation

A

f = 1/T

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36
Q

dispersion

A

when waves separate out due to a wave travelling through a different medium (different wavelengths travel at different speeds)

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37
Q

intensity equation

A

I = P/A

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38
Q

intensity distance relationship

A

inverse square law

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39
Q

intensity

A

I

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40
Q

speed of sound in air

A

340 m/s

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41
Q

speed of light in a vacuum

A

3e8m

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42
Q

phase difference

A

how much one wave is in front or behind another wave

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43
Q

Transverse wave

A

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

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44
Q

rarefraction

A

oscillations are far apart

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45
Q

compression

A

oscillations are close together

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46
Q

how does a graph show transverse waves

A

displacement distance graph

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47
Q

How does a graph show longitudinal waves

A

displacement time graph

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48
Q

what type of wave an EM wave

A

transverse waves

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49
Q

wavefront

A

the leading edge of one complete wave

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50
Q

coherence

A

same frequency + unchanging phase difference

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51
Q

superposition

A

The resultant displacement can be found by adding the two displacements together from interfering waves

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52
Q

when does superposition occur?

A

for all waves

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53
Q

superposition of coherent waves

A

provides a constant pattern of interference

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54
Q

path difference

A

the difference in distance traveled by the two waves from their respective sources to a given point on the pattern

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55
Q

difference between phase difference and path difference

A

phase difference is worked out by path difference. There could be zero phase difference but still have a path difference.

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56
Q

What is the structure of an EM wave?

A

electric and magnetic fields which oscillate in phase and are perpendicular to each other

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57
Q

progressive wave characteristics

A

transfers energy

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58
Q

standing/stationary wave characteristics

A

stores energy

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59
Q

how are stationary waves formed in a string?

A

the wave reflects back from a terminator and interferes with itself

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60
Q

resonant frequencies

A

a natural frequency of vibration determined by the physical parameters of the vibrating object.

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61
Q

harmonics

A

a wave where its frequency is a multiple of the material natural frequency resulting in a standing wave

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62
Q

Where are nodes on a standing wave in a string

A

at the end of the string (+in between depending on the harmonic)

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63
Q

mass per unit length

A

mass of an object divided by its length

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64
Q

how to calculate wave speed of a standing wave on a string

A

V = √(T/μ) where μ is the mass per unit length

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65
Q

how is a standing wave formed inside a closed pipe?

A

blowing an air column down a closed pipe results in it being reflected back up. The two waves superpose to form a stationary longitudinal wave.

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66
Q

How is stationary waves in a pipe drawn in a diagram?

A

drawn as a displacement distance graph

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67
Q

where is a node formed in a closed pipe standing wave

A

at the closed end

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68
Q

why is a node formed at the closed end of a pipe

A

the air cannot oscillate freely

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69
Q

Where is an antinode formed in a closed pipe

A

at the open end

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70
Q

whats different about closed pipe harmonics

A

it can only form odd harmonics

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71
Q

Where is a anti node formed in an open pipe

A

at both ends (because they’re open)

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72
Q

wave diagram

A

shows the wave fronts (straight lines perpendicular to direction of travel)

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73
Q

ray diagram

A

shows a single ray and the direction and action of a wave

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74
Q

reflection

A

the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated

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75
Q

laws of reflections (2)

A

when light is reflected

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76
Q

angles in reflection

A

the angle i between the incident ray and the normal is the same as the angle r between the reflected ray and the normal

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77
Q

refraction

A

the change in direction of wave propagation due to it moving through a different medium

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78
Q

why does refraction occur

A

waves travel at different speeds in different mediums.

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79
Q

less dense -> more dense

A

how does light bend

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80
Q

snells law

A

n = sini/sinr = c/v

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81
Q

refractive index letter

A

n

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82
Q

absolute refractive index

A

a ratio of the speed of light in a vacuum to the speed of light in a given medium

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83
Q

1n2 =

A

calculating the refractive index between two materials

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84
Q

critical angle

A

the largest angle at which refraction out of a denser medium is possible

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85
Q

refraction between two mediums equation

A

n1sin θ1= n2 sin θ2

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86
Q

how do you calculate the critical angle?

A

by making θ2 90 degrees in the equation: n1sin θ1= n2 sin θ2

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87
Q

absolute refractive index of air

A

1

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88
Q

absolute refractive index of water (use to check calculations)

A

1.33

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89
Q

total internal reflection

A

the complete reflection of a wave where the angle of incidence exceeds the critical angle

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90
Q

if i is less than the critical angle then

A

refraction

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91
Q

if i = critical angle =>

A

partial TIR (multiple rays)

92
Q

if i>critical angle =>

A

TIR

93
Q

how to measure the refractive index of a solid material

A

use a glass block to shine light through and trace the path

94
Q

focal length

A

the distance between the optical centre and the principle focus

95
Q

diverging lens

A

a concave lens

96
Q

converging lens

A

a bulging lens

97
Q

convex lens

A

converging

98
Q

concave lens

A

diverging

99
Q

ray bending in a converging lens

A

going in: bends towards the normal

100
Q

if the object is between the focal length and a converging lens then the image is

A

magnified

101
Q

if the object is beyond the focal length of a converging lens then the image is

A

magnified

102
Q

virtual principal focus

A

is you trace back the diverged rays to a single point

103
Q

power of a diverging lens

A

always negative

104
Q

What does diverging lens do to the image

A

diminished

105
Q

what does the focal length depend on

A

the curvature of the surface and the material used

106
Q

the more powerful the lens the…

A

shorter the focal length

107
Q

power of a lens equation

A

P = 1/f

108
Q

lens equation

A

1/f = 1/u + 1/v distances to real objects and images are positive

109
Q

magnification equation

A

magnification = image distance/ object distance

110
Q

real image

A

an image that can be projected onto a screen

111
Q

virtual image

A

an image that can’t be projected onto a screen (appears to come from behind the lens)

112
Q

combining lens powers

A

P = P1 + P2 + P3… for thin lenses

113
Q

ray diagram for converging lens (object beyond focal length)

A

draw a horizontal line from the top of the object to the y axis then down through the focal point on the opposite side

114
Q

ray diagram for converging lens (object between focal point and lens)

A

draw a horizontal line from the top of the object to the y axis then down through the focal point on the opposite side

115
Q

ray diagram for diverging lens

A

draw a horizontal line from the top of the object to the y axis then align the rule with the focal point on the same side of the lens and draw a line up from the point of the y axis

116
Q

not polarised

A

wave oscillates in all directions

117
Q

plane polarised

A

wave oscillates in one plane only

118
Q

plane polarised examples

A

scattered/reflected light

119
Q

polarising longitudinal waves

A

can’t be done

120
Q

crosses polarised

A

when filters are perpendicular to each other so no light can get through

121
Q

diffraction

A

the spreading out of a wave as it goes past an obstacle or through a gap

122
Q

When a wave passes through a gap that is a similar size to their wavelength…

A

there is a lot of diffraction

123
Q

monochromatic

A

only one wavelength

124
Q

interference pattern

A

a series of maximum and minimum points that can be seen on a screen from interfering coherent waves

125
Q

nλ =

A

dsinθ

126
Q

d =

A

slit spacing

127
Q

q

A

a

128
Q

how to calculate d from lines per m

A

n = 1/(lines per m)

129
Q

What did Planck work on?

A

He looked at black body radiation; He theorised that radiation was emitted in discrete packets of energy; He found there was a link between energy and frequency

130
Q

What is a Quanta?

A

Discrete packets of energy

131
Q

Planck’s equation

A

E=hf

132
Q

h value

A

6.633e-34

133
Q

What did Einstein theorise?

A

That concentrated packets of energy had particle-like properties and were called photons

134
Q

Photon

A

Concentrated discrete packets of energy which have particle-like properties

135
Q

What is the EM spectrum from a particle point of view?

A

Many photons with different levels of energy

136
Q

How much do photons weigh?

A

Weightless

137
Q

How can photons travel at the speed of light?

A

Because they’re weightless

138
Q

What letter represents the speed of light?

A

c

139
Q

How is the equation E = hc/λ formed?

A

Combining E=hf and c=fλ

140
Q

Electron volt

A

One electronvolt is the energy gained by an electron when it is accelerated through a p.d. of 1v (W= QV)

141
Q

How to convert joules to eV

A

Divide by 1.6x10^-19

142
Q

How to convert eV to Joules

A

Multiply by 1.6x10^-19

143
Q

How to find Planck’s constant?

A

Set up a potential divider circuit with a parallel section with different coloured LEDs, an ammeter and a voltmeter. Measure the voltage and record the wavelength (read from packet). Plot a graph of V against 1/λ. The gradient equals Vλ. Substitute E = eV into E = hc/λ, input gradient value and rearrange to get h.

144
Q

Who worked out the photoelectric effect?

A

Einstein

145
Q

What is the photoelectric effect?

A

The emission of electrons from the surface of, generally, a metal in response to incident light.

146
Q

What shows the photoelectric effect?

A

When a charge is given to an electroscope, they repel each other, so the gold leaf will lift and move away from the metal pole.

147
Q

How can the charge of an electroscope be found?

A

The angle the gold leaf lifts to

148
Q

Why does the wave model not back up the photoelectric effect?

A

All the frequencies should combine energy to liberate the electrons

149
Q

How many photons can liberate a single electron?

A

1

150
Q

If wavelength increases…

A

Frequency decreases; therefore, electrons have less kinetic energy and eventually none are liberated.

151
Q

If wavelength decreases

A

Frequency increases; therefore, electrons have more kinetic energy.

152
Q

If intensity increases

A

More electrons are released, but with the same kinetic energy. If it is below the threshold frequency, intensity has NO effect.

153
Q

Electrons are trapped inside __________ and in order to escape it has to ________

A

Energy wells; absorb enough energy

154
Q

How does the material affect the energy well?

A

Different sizes, therefore different amounts of energy are needed to liberate the electrons

155
Q

Work function

A

The amount of energy needed for the electrons to escape their energy well

156
Q

Which formula works out the work function

A

hf = Φ + E.K. max

157
Q

If the electron is given just enough energy to release from the energy well, its kinetic energy equals 0 therefore…

A

Threshold frequency can be found by Φ/h

158
Q

It doesn’t matter how many IR photons land on the metal… if

A

All of them are below the threshold frequency, no single electron will be liberated

159
Q

Photoelectron

A

A liberated electron

160
Q

Intensity is proportional to

A

Rate of emission of photoelectrons

161
Q

Broglie said that for

A

Any particle that had momentum, it also has wavelength λ = h/p

162
Q

Relativistic mass

A

As a particle gets closer to the speed of light, the mass tends to increase due to relativistic effects

163
Q

The intensity of a wave at a point represents

A

The probability of a wave being there

164
Q

The electrons have _____ different energy levels but its energy is _______

A

Infinite; finite

165
Q

How do you work out the energy changes of an atom?

A

Calculate the frequency and wavelength needed to give the energy to move up levels and equally how much is emitted when it falls back down levels

166
Q

Emission spectra

A

Shows the certain wavelengths of photons which are given off by an element after it is excited and the electrons drop back down to their original energy levels and emit energy

167
Q

Absorption spectra

A

Where certain frequencies of light are missing because they’re being absorbed by that element

168
Q

Threshold frequency

A

The lowest frequency of light at which electrons are still released from a surface

169
Q

What experiment determines the work function of different materials and the value of h?

A

Stopping voltage experiment

170
Q

What does the graph from the stopping voltage experiment show?

A

Gradient = h; F0 (x-intercept) = threshold frequency; y-intercept = work function

171
Q

What does the y-intercept from the stopping voltage experiment show?

A

The voltage needed to stop an electron being liberated by light of 0 frequency and so 0 energy (the work function)

172
Q

What axes are plotted from the stopping voltage experiment?

A

y = stopping voltage; x = frequency

173
Q

If the p.d. in a stopping voltage experiment is increased, what happens?

A

Electrons are accelerated faster as they move in the same direction as the current

174
Q

If the p.d. in a stopping voltage experiment is decreased, what happens?

A

The battery is more effective than the photoelectric effect, therefore the electrons are slowed and start to move backwards.

175
Q

What is stopping voltage?

A

The voltage at which the battery becomes more powerful than the photoelectric effect and the electrons are slowed

176
Q

Why are electrons only emitted about a threshold frequency?

A

Photon energy is proportional to frequency; therefore photon energy must be greater than the work function to liberate an electron. All the energy must come from a single photon.

177
Q

Line spectra

A

Specific frequencies/wavelengths show the absorption/emission lines within a narrow band of wavelengths

178
Q

How do line spectra provide evidence for the existence of energy levels in atoms?

A

Photons associated with particular energies show electron transitions up and down the discrete energy levels

179
Q

Wave model features

A

Diffraction, refraction, reflection, have a frequency, interfere with each other, pass through each other

180
Q

Photon model features (features of particles)

A

Have mass, reflect, experience forces between each other, have volume, can have charge, have momentum, have density

181
Q

The shorter the pulse…

A

The shorter the distance that can be measured

182
Q

Why does the photon model work for the photoelectric effect?

A

The energy of one photon is used to liberate one electron, meaning the threshold frequency must be high enough. The energy is proportional to the frequency and any energy greater than the work function is transferred to the electron as kinetic energy.

183
Q

Why does the wave model not work for the photoelectric effect?

A

Frequency would build up to high enough to liberate an electron; K.E. would depend on the intensity of the light.

184
Q

Long wavelength photon means…

A

Less energy levels moved up

185
Q

High frequency photon means…

A

The more energy levels it jumps up

186
Q

How is a photon emitted?

A

Electrons don’t remain in an excited state, so they de-excite and drop down to the ground state and emit energy in the form of a photon

187
Q

How can electrons be excited?

A

If a photon is absorbed; if electrons are hit by other electrons

188
Q

Energy delivered by photon (hf) =

A

Difference between the energy levels

189
Q

Ground state

A

The lowest energy level where electrons are usually found

190
Q

Why are only certain frequencies absorbed by atoms?

A

Electrons can only exist in discrete energy levels

191
Q

Kinetic energy gained by accelerating electron through a potential difference =

A

eV

192
Q

The amount of diffraction that a wave undergoes depends on the

A

Amplitude of the incident wave and the size of the opening

193
Q

Experiment to show that electrons behave as a wave

A

Direct the electrons through a crystal. If the size of the crystal atom is similar to the wavelength of the electron, it diffracts (a wave property)

194
Q

Why is diffraction more obvious with sound than light?

A

Sound has a longer wavelength so it occurs more at our scale

195
Q

Intensity of light through two Polaroids is greatest when

A

The Polaroids are parallel

196
Q

hf

A

Energy of a photon

197
Q

Ф

A

The energy required from a single photon to release an electron from its energy well (work function)

198
Q

Kinetic energy of photoelectrons depends on…

A

The frequency of the incident photon

199
Q

More intense light means…

A

More photoelectrons released (IF FREQUENCY OVER THRESHOLD FREQUENCY)

200
Q

What does the number of maxima correspond to?

A

The highest integer value of d/λ is the number of maxima on one side of the central order (not including the central order)

201
Q

Why are certain frequencies missing from an absorption spectrum?

A

Electrons get excited by absorbing photons. Electrons have fixed energy levels. Only certain transitions are possible, so only certain photon energies are absorbed, so some frequencies are missing. The set of frequencies absorbed depends on the element.

202
Q

Energy of photon absorbed =

A

Difference in energy levels

203
Q

Range of visible light wavelengths

A

400nm - 700nm

204
Q

How to get the first order maxima closer together?

A

Increase the frequency of the laser

205
Q

As speed decreases…

A

Wavelength decreases

206
Q

Wave property which only applies to transverse waves?

A

Polarisation

207
Q

Not polarised

A

Oscillates in all the planes perpendicular to the direction of travel

208
Q

Standing wave

A

A series of nodes and antinodes formed for interfering coherent waves

209
Q

Out of phase value in radians

A

pi

210
Q

In phase value

A

0, 2 pi

211
Q

Fundamental frequency

A

Lowest frequency of a standing wave that can be set

212
Q

Frequency of ultrasound

A

20,000 Hz

213
Q

Image from a lens where the object is beyond 2x the focal length

A

Inverted, diminished, real

214
Q

Object on focal point convex lens

A

Rays are parallel, no image will be formed

215
Q

Long sight

A

The power is too weak so it doesn’t converge on the retina

216
Q

Short sight

A

Too powerful, the image converges before the retina

217
Q

Millikan’s experiment

A

They let small drops of oil fall between the two plates. By adjusting the p.d. between the plates, the forces were balanced on the drop. mg = vQ/d to work out Q. To find m they needed to measure the radius. They let it move at terminal velocity and used forces to find r.

218
Q

Millikan’s experiment conclusion

A

Measured the charge of an oil drop to be always a multiple of 1.6e-19, so he deduced the charge of an electron is 1.6e-19 C

219
Q

Millikan’s experiment set up

A

Electric field, atomiser to spray oil drops into the electric field, microscope to view the oil drops

220
Q

How did they find r in Millikan’s experiment?

A

Let it fall at terminal velocity and then used forces; 6πνrv = mg = 4/3ρgπr^3

221
Q

Frequency from number of oscillations in a given time

A

f = number of oscillations / time

222
Q

Line spectra

A

Specific narrow band of frequencies or wavelengths that an element has absorbed/emitted

223
Q

What does firing electrons through a screen show about their nature?

A

Electrons spread out and form an interference pattern; Electrons must behave as waves (their wavelength is a similar size to the spacing)

224
Q

What does vertically polarised mean?

A

Light only oscillates in the vertical plane perpendicular to the direction of travel

225
Q

Why can two oppositely polarised sources not interfere?

A

They oscillate perpendicular to each other, so the opposite/same components cannot undergo superposition

226
Q

Explain how refraction is caused (2 parts)

A

Materials are different densities; Light changes direction and appears to come from another point

227
Q

Critical angle

A

The angle of incidence for light travelling from a denser medium that has an angle of refraction of 90

Physics The Good One (16 decks)

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