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
2
Q
What is a wavelength?
A
the distance between two matching points in neighbouring waves
3
Q
amplitude
A
the maximum displacement a point moves from its centre of oscillation
4
Q
The larger the amplitude…
A
the greater the energy
5
Q
period
A
the time taken for a point on a wave to move through one complete oscillation
6
Q
frequency
A
the number of oscillations per second
7
Q
frequency =
A
1/time period
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.
9
Q
higher frequency means…
A
more energy
10
Q
what is the order of the electromagnetic spectrum?
A
radio ( longest wavelength, least freq)
Microwaves
Infared
Visible light
Ultra violet
X rays
Gamma (shortest wavelength, highest frequency)
11
Q
what is the wavelength of a radio wave?
A
10^3 - 10^1 m
12
Q
what is the wavelength of a microwave?
A
10^-2 m
13
Q
what is the wavelength of a Infra-red wave?
A
10^-5 m
14
Q
what is the wavelength of a visible light wave?
A
10^-7 m
15
Q
what is the wavelength of a ultra violet wave?
A
10^-8 m
16
Q
what is the wavelength of a x-ray wave?
A
10^-10 m
17
Q
what is the wavelength of a gamma ray?
A
10^-12 m (+)
18
Q
What is diffraction?
A
is the spreading out of a wave as it goes past an obstacle or through a gap
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
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.
21
Q
constructive interference
A
is known as in phase
They supervise each other where peaks aligns causing the amplitude to be both peak amplitudes adding to each other
22
Q
What is superposition?
A
when the wave amplitudes are added
23
Q
destructive interference
A
is known as out of phase
(In anti phase)
When one waves peak Aligns with another’s trough causing the amplitude to be 0
24
Q
degrees to radians conversion
A
radians = (degrees * pi)/180
25
radians to degrees conversion
degrees = (radians * 180) / pi
26
For two waves of light to be coherent the waves must
originate from one source
27
how does an additional converging lens affect the eye
decreases the image distance as the lens adds more power
28
virtual image
cannot be projected onto a screen
29
unit of viscosity
PaS
30
why does intensity decrease over distance
the area the wave is spread out over is larger so the intensity is lower (interference can also affect the intensity)
31
wave speed
the speed the wave travels. v = fλ
32
wave equation
v = fλ
33
wave
something that transfers energy from one point to another
34
describe a longitudinal wave
particles in metal vibrate along direction of propagation making compressions and rarefactions
35
frequency time period equation
f = 1/T
36
dispersion
when waves separate out due to a wave travelling through a different medium (different wavelengths travel at different speeds)
37
intensity equation
I = P/A
38
intensity distance relationship
inverse square law
39
intensity
I
40
speed of sound in air
340 m/s
41
speed of light in a vacuum
3e8m
42
phase difference
how much one wave is in front or behind another wave
43
Transverse wave
a type of wave in which particles oscillate perpendicular to the direction of energy transfer in the wave
44
rarefraction
regions of low pressure due to particles being spread further apart.
45
compression
regions of high pressure due to particles being close together
46
how does a graph show transverse waves
displacement distance graph
47
How does a graph show longitudinal waves
displacement time graph
48
what type of wave an EM wave
transverse waves
49
wavefront
the leading edge of one complete wave
50
coherence
same frequency + unchanging phase difference
51
superposition
The resultant displacement can be found by adding the two displacements together from interfering waves
52
when does superposition occur?
2 waves are in phase
53
superposition of coherent waves
provides a constant pattern of interference
54
path difference
the difference in distance traveled by the two waves from their respective sources to a given point on the pattern
55
difference between phase difference and path difference
phase difference is worked out by path difference. There could be zero phase difference but still have a path difference.
56
What is the structure of an EM wave?
electric and magnetic fields which oscillate in phase and are perpendicular to each other
57
progressive wave characteristics
transfers energy
58
standing/stationary wave characteristics
stores energy
59
how are stationary waves formed in a string?
the wave reflects back from a terminator and interferes with itself
60
resonant frequencies
a natural frequency of vibration determined by the physical parameters of the vibrating object.
61
harmonics
a wave where its frequency is a multiple of the material natural frequency resulting in a standing wave
62
Where are nodes on a standing wave in a string
at the end of the string (+in between depending on the harmonic)
63
mass per unit length
mass of an object divided by its length
64
how to calculate wave speed of a standing wave on a string
V = √(T/μ) where μ is the mass per unit length
65
how is a standing wave formed inside a closed pipe?
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.
66
How is stationary waves in a pipe drawn in a diagram?
drawn as a displacement distance graph
67
where is a node formed in a closed pipe standing wave
at the closed end
68
why is a node formed at the closed end of a pipe
the air cannot oscillate freely
69
Where is an antinode formed in a closed pipe
at the open end
70
whats different about closed pipe harmonics
it can only form odd harmonics
71
Where is a anti node formed in an open pipe
at both ends (because they’re open)
72
wave diagram
shows the wave fronts (straight lines perpendicular to direction of travel)
73
ray diagram
shows a single ray and the direction and action of a wave
74
reflection
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
75
laws of reflections (2)
the angle of incidence is equal to the angle of reflection. When a light ray strikes a surface, it can either be absorbed, transmitted or reflected.
76
angles in reflection
the angle i between the incident ray and the normal is the same as the angle r between the reflected ray and the normal
77
refraction
the change in direction of wave propagation due to it moving through a different medium
78
why does refraction occur
waves travel at different speeds in different mediums.
79
less dense -> more dense
bends towords the normal as light slows down
80
snells law
n = sini/sinr = c/v
81
refractive index letter
n
82
absolute refractive index
a ratio of the speed of light in a vacuum to the speed of light in a given medium
83
1n2 =
calculating the refractive index between two materials
84
critical angle
the largest angle at which refraction out of a denser medium is possible
85
refraction between two mediums equation
n1sin θ1= n2 sin θ2
86
how do you calculate the critical angle?
by making θ2 90 degrees in the equation: n1sin θ1= n2 sin θ2
87
absolute refractive index of air
1
88
absolute refractive index of water (use to check calculations)
1.33
89
total internal reflection
the complete reflection of a wave where the angle of incidence exceeds the critical angle
90
if i is less than the critical angle then
refraction
91
if i = critical angle =>
partial TIR (multiple rays)
92
if i>critical angle =>
TIR
93
how to measure the refractive index of a solid material
use a glass block to shine light through and trace the path
94
focal length
the distance between the optical centre and the principle focus
95
diverging lens
a concave lens
96
converging lens
a bulging lens
97
convex lens
converging
98
concave lens
diverging
99
ray bending in a converging lens
going in: bends towards the normal
100
if the object is between the focal length and a converging lens then the image is
magnified
101
if the object is beyond the focal length of a converging lens then the image is
magnified
102
virtual principal focus
is you trace back the diverged rays to a single point
103
power of a diverging lens
always negative
104
What does diverging lens do to the image
diminished
105
what does the focal length depend on
the curvature of the surface and the material used
106
the more powerful the lens the…
shorter the focal length
107
power of a lens equation
P = 1/f
108
lens equation
1/f = 1/u + 1/v distances to real objects and images are positive
109
magnification equation
magnification = image distance/ object distance
110
real image
an image that can be projected onto a screen
111
virtual image
an image that can’t be projected onto a screen (appears to come from behind the lens)
112
combining lens powers
P = P1 + P2 + P3… for thin lenses
113
ray diagram for converging lens (object beyond focal length)
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
ray diagram for converging lens (object between focal point and lens)
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
ray diagram for diverging lens
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
not polarised
wave oscillates in all directions
117
plane polarised
wave oscillates in one plane only
118
plane polarised examples
scattered/reflected light
119
polarising longitudinal waves
can’t be done
120
crosses polarised
when filters are perpendicular to each other so no light can get through
121
diffraction
the spreading out of a wave as it goes past an obstacle or through a gap
122
When a wave passes through a gap that is a similar size to their wavelength…
there is a lot of diffraction
123
monochromatic
only one wavelength
124
interference pattern
a series of maximum and minimum points that can be seen on a screen from interfering coherent waves
125
nλ =
dsinθ
126
d =
slit spacing
127
q
a
128
how to calculate d from lines per m
n = 1/(lines per m)
129
What did Planck work on?
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
What is a Quanta?
Discrete packets of energy
131
Planck's equation
E=hf
132
h value
6.633e-34
133
What did Einstein theorise?
That concentrated packets of energy had particle-like properties and were called photons
134
Photon
Concentrated discrete packets of energy which have particle-like properties
135
What is the EM spectrum from a particle point of view?
Many photons with different levels of energy
136
How much do photons weigh?
Weightless
137
How can photons travel at the speed of light?
Because they're weightless
138
What letter represents the speed of light?
c
139
How is the equation E = hc/λ formed?
Combining E=hf and c=fλ
140
Electron volt
One electronvolt is the energy gained by an electron when it is accelerated through a p.d. of 1v (W= QV)
141
How to convert joules to eV
Divide by 1.6x10^-19
142
How to convert eV to Joules
Multiply by 1.6x10^-19
143
How to find Planck's constant?
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 c/λ,
input gradient value
rearrange to get h.
144
Who worked out the photoelectric effect?
Einstein
145
What is the photoelectric effect?
The emission of electrons from the surface of, generally, a metal in response to incident light.
146
What shows the photoelectric effect?
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
How can the charge of an electroscope be found?
The angle the gold leaf lifts to
148
Why does the wave model not back up the photoelectric effect?
All the frequencies should combine energy to liberate the electrons
149
How many photons can liberate a single electron?
1
150
If wavelength increases…
Frequency decreases; therefore, electrons have less kinetic energy and eventually none are liberated.
151
If wavelength decreases
Frequency increases; therefore, electrons have more kinetic energy.
152
If intensity increases
More electrons are released, but with the same kinetic energy. If it is below the threshold frequency, intensity has NO effect.
153
Electrons are trapped inside __________ and in order to escape it has to ________
Energy wells; absorb enough energy
154
How does the material affect the energy well?
Different sizes, therefore different amounts of energy are needed to liberate the electrons
155
Work function
The amount of energy needed for the electrons to escape their energy well
156
Which formula works out the work function
hf = Φ + E.K. max
157
If the electron is given just enough energy to release from the energy well, its kinetic energy equals 0 therefore…
Threshold frequency can be found by Φ/h
158
It doesn’t matter how many IR photons land on the metal… if
All of them are below the threshold frequency, no single electron will be liberated
159
Photoelectron
A liberated electron
160
Intensity is proportional to
Rate of emission of photoelectrons
161
Broglie said that for
Any particle that had momentum, it also has wavelength λ = h/p
162
Relativistic mass
As a particle gets closer to the speed of light, the mass tends to increase due to relativistic effects
163
The intensity of a wave at a point represents
The probability of a wave being there
164
The electrons have _____ different energy levels but its energy is _______
Infinite; finite
165
How do you work out the energy changes of an atom?
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
Emission spectra
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
Absorption spectra
Where certain frequencies of light are missing because they’re being absorbed by that element
168
Threshold frequency
The lowest frequency of light at which electrons are still released from a surface
169
What experiment determines the work function of different materials and the value of h?
Stopping voltage experiment
170
What does the graph from the stopping voltage experiment show?
Gradient = h; F0 (x-intercept) = threshold frequency; y-intercept = work function
171
What does the y-intercept from the stopping voltage experiment show?
The voltage needed to stop an electron being liberated by light of 0 frequency and so 0 energy (the work function)
172
What axes are plotted from the stopping voltage experiment?
y = stopping voltage; x = frequency
173
If the p.d. in a stopping voltage experiment is increased, what happens?
Electrons are accelerated faster as they move in the same direction as the current
174
If the p.d. in a stopping voltage experiment is decreased, what happens?
The battery is more effective than the photoelectric effect, therefore the electrons are slowed and start to move backwards.
175
What is stopping voltage?
The voltage at which the battery becomes more powerful than the photoelectric effect and the electrons are slowed
176
Why are electrons only emitted about a threshold frequency?
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
Line spectra
Specific frequencies/wavelengths show the absorption/emission lines within a narrow band of wavelengths
178
How do line spectra provide evidence for the existence of energy levels in atoms?
Photons associated with particular energies show electron transitions up and down the discrete energy levels
179
Wave model features
Diffraction, refraction, reflection, have a frequency, interfere with each other, pass through each other
180
Photon model features (features of particles)
Have mass, reflect, experience forces between each other, have volume, can have charge, have momentum, have density
181
The shorter the pulse…
The shorter the distance that can be measured
182
Why does the photon model work for the photoelectric effect?
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
Why does the wave model not work for the photoelectric effect?
Frequency would build up to high enough to liberate an electron; K.E. would depend on the intensity of the light.
184
Long wavelength photon means…
Less energy levels moved up
185
High frequency photon means…
The more energy levels it jumps up
186
How is a photon emitted?
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
How can electrons be excited?
If a photon is absorbed; if electrons are hit by other electrons
188
Energy delivered by photon (hf) =
Difference between the energy levels
189
Ground state
The lowest energy level where electrons are usually found
190
Why are only certain frequencies absorbed by atoms?
Electrons can only exist in discrete energy levels
191
Kinetic energy gained by accelerating electron through a potential difference =
eV
192
The amount of diffraction that a wave undergoes depends on the
Amplitude of the incident wave and the size of the opening
193
Experiment to show that electrons behave as a wave
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
Why is diffraction more obvious with sound than light?
Sound has a longer wavelength so it occurs more at our scale
195
Intensity of light through two Polaroids is greatest when
The Polaroids are parallel
196
hf
Energy of a photon
197
Ф
The energy required from a single photon to release an electron from its energy well (work function)
198
Kinetic energy of photoelectrons depends on…
The frequency of the incident photon
199
More intense light means…
More photoelectrons released (IF FREQUENCY OVER THRESHOLD FREQUENCY)
200
What does the number of maxima correspond to?
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
Why are certain frequencies missing from an absorption spectrum?
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
Energy of photon absorbed =
Difference in energy levels
203
Range of visible light wavelengths
400nm - 700nm
204
How to get the first order maxima closer together?
Increase the frequency of the laser
205
As speed decreases…
Wavelength decreases
206
Wave property which only applies to transverse waves?
Polarisation
207
Not polarised
Oscillates in all the planes perpendicular to the direction of travel
208
Standing wave
A series of nodes and antinodes formed for interfering coherent waves
209
Out of phase value in radians
pi
210
In phase value
0, 2 pi
211
Fundamental frequency
Lowest frequency of a standing wave that can be set
212
Frequency of ultrasound
20,000 Hz
213
Image from a lens where the object is beyond 2x the focal length
Inverted, diminished, real
214
Object on focal point convex lens
Rays are parallel, no image will be formed
215
Long sight
The power is too weak so it doesn’t converge on the retina
216
Short sight
Too powerful, the image converges before the retina
217
Millikan's experiment
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
Millikan's experiment conclusion
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
Millikan's experiment set up
Electric field, atomiser to spray oil drops into the electric field, microscope to view the oil drops
220
How did they find r in Millikan's experiment?
Let it fall at terminal velocity and then used forces; 6πνrv = mg = 4/3ρgπr^3
221
Frequency from number of oscillations in a given time
f = number of oscillations / time
222
Line spectra
Specific narrow band of frequencies or wavelengths that an element has absorbed/emitted
223
What does firing electrons through a screen show about their nature?
Electrons spread out and form an interference pattern; Electrons must behave as waves (their wavelength is a similar size to the spacing)
224
What does vertically polarised mean?
Light only oscillates in the vertical plane perpendicular to the direction of travel
225
Why can two oppositely polarised sources not interfere?
They oscillate perpendicular to each other, so the opposite/same components cannot undergo superposition
226
Explain how refraction is caused (2 parts)
Materials are different densities; Light changes direction and appears to come from another point
227
Critical angle
The angle of incidence for light travelling from a denser medium that has an angle of refraction of 90
228
229
230
