Topic 5 - Waves / Particle Nature of Light Flashcards
1
Q
longitudinal waves
A
a type pf wave in which the particles oscillate parallel to the direction of the wave
2
Q
wavelength
A
the distance between two matching points in neighbouring waves, measured in metres (m)
3
Q
amplitude
A
the maximum displacement a point moves from its centre of oscillation, measure in metres (m)
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, measures in seconds (s)
6
Q
frequency
A
the number of oscillations per second, measured in Hertz (Hz)
OR
the number of waves that pass a point in one second, measured in Hertz (Hz)
7
Q
freqeuncy =
A
1/time period
8
Q
describe electronmagnetic 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 electronmagnetic spectrum?
A
radio, micro, IR, visible, UV, x-ray, Gamma
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
diffraction
A
is the spreading out of a wave as it goes past an obstacle or through a gap
19
Q
What is Huygens principal?
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 interfence
A
is known as in phase, where a trough and trough meet or a peak and a peak meet. the waves have the same frequency and wavelength but double(the addition of the two waves) the amplitude.
22
Q
What is superposition?
A
when the wave amplitudes are added
23
Q
destructive interference
A
is known as out of phase, where a trough of one wave meets a peak of another wave the waves must have the phase difference of 180 degrees. the waves cancel each other out.
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 a additional converging lens effect 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 effect 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 rarefractions
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
the rate of flow of energy per unit area perpendicular to the direction of travel of the wave
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
oscillations are far apart
45
compression
oscillations are close together
46
how does a graph show transverse waves
displacement distance graph
| displacement shows amplitude
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?
for all waves, even if they're no coherent.
53
superposition of coherent waves
provides a constant patter 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
- each point will reach the same amplitude
- each particle oscillates over the same path but there is a phase lag between each particle
58
standing/stationary wave charateristics
- stores energy
- amplitude varies
- between two nodes all the particles oscillate in phase; on either side of a node there are outp of phase
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 it length, the thickness of string effects this
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, so it appears as a transverse wave
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
show 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)
- when light is reflected, the incident ray, the reflected ray and the normal all lie inside the same plane
- the angle i between the incident ray and the normal is the same as the angle r between the reflected ray and the normal
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
| how does light bend
towards the normal
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
n2/n1
| where n2 and n1 are the absolute refractive indexes of each material
84
critical angle
the largest angle at which refractuib 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
n = 1/sinC
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 =>
particial 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
| going out: bends away from the normal
100
if the object is between the focal length and a converging lens then the image is
- magnified
- upright
- virtual image
101
if the object is beyond the focal length of a converging lens then the image is
- magnified
- inverted
- real image
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
- upright
- virtual image
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 postive
distances to virtual images are negative
focal length of converging is positive, focal length of diverging is negative
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
combing lens powers
P = P1 + P2 + P3...
| for thin lenses
113
ray diagram for converging lens (object beyond focal length)
1. draw a horizontal line from the top of the object to the y axis then down through the focal point on the opposite side
2. draw a line directly through the centre of the axes from the top of the object
3. draw a line through the focal point on the same side of the lens, when it hits the y axis go horizontally across. Where all three lines cross is the top of the image.
114
ray diagram for converging lens (object between focal point and lens)
1. draw a horizontal line from the top of the object to the y axis then down through the focal point on the opposite side
2. draw a line directly through the centre of the axes from the top of the object
3. from the two sloped lines dot each one back. Where they cross is the top of the object
115
ray diagram for diverging lens
1. 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
2. draw a line directly through the centre of the axes from the top of the object
3. trace back the upward sloped line, where it crosses the downward diagonal is where the top of the image is
116
not polarised
wave oscillates in all directions
117
plane polarsied
wave oscillates in one plane only
118
plane polarised examples
scattered/reflected light, microwave and radiowave sources
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 wave length...
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
how to calculate d from lines per m
n = 1/(lines per m)
128
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
129
What is a Quanta?
discrete packets of energy
130
plancks equation
E=hf
131
h value
6.633e-34
132
What did einstein theorise?
That concentrated packets of energy had particle like properties and were called photons
133
photon
concentrated discrete packets of energy which have particle like properties
134
What is the EM spectrum from a particle point of view?
many photons with different levels of energy
135
How much do photons weigh?
weightless
136
how can photons travel at the speed of light?
because theyre weightless
137
what letter represents the speed of light?
c
138
how is the equation E = hc/ lambda formed?
combining E=hf and c=fλ
139
Electron volt
One electronvolt is the energy gained by an electron when it is accelerated through a p.d. of 1v (W= QV)
140
how to convert joules to eV
divide by 1.6x10^-19
141
how to convert eV to Joules
multiply by 1.6x10^-19
142
how to find plancks constant?
- set up a potential divider circuit with a paralell 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 agaisnt 1/λ
- the gradient equals Vλ
- substitute E = eV into E =hc/λ input gradint value and rearrange to get h
143
Who worked out the photoelectric effect?
Einstein
144
What is the photoelectric effect?
the emission of electrons from the surface of, generally, a metal in response to incident light.
145
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.
146
How can the charge of an electroscope be found?
the angle the gold leaf lifts too
147
why does the wave model no backup the photoelectric effect?
all the frequencies should combine energy to liberate the electrons
148
how many photons can liberate a single electron?
1
149
if wavelength increases...
frequency decreases therefore electrons have less kinetic energy and eventually none are liberated
150
if wavelength decrease
frequency increases therefore electrons have more kinetic energy
151
if intensity increases
more electrons are increased but with the same kinetic energy. if it is below the threshold frequency intensity has NO effect
152
electrons are trapped inside __________ and in order to escape it has to _________
energy wells
| absorb enough energy
153
How does the material effect the energy well?
different sizes therefore different amounts of energy are needed to liberate the electrons
154
work function
the amount of energy needed for the electrons to escape their energy well
155
which formula works out the work function
hf = Φ + E.K. max
156
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
157
it doesn't matter how many IR photons land on the metal... if
all of them are below the freshold frequency no single electron will be liberated
158
photoelectron
a liberated electron
159
intensity is proportional to
rate of emmision of photoelectrons
160
Broglie said that for
any particle that had momentum it also has wavelength λ = h/p
161
relativistic mass
as a particle gets closer to the speed of light the mass tends to increase due to relativistic effects
162
The intensity of a wave at a point represents
the probability of a wave being there
163
the electrons have _____ different energy levels by its energy is _______
infinite
| finite
164
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
165
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 there original energy levels ad emit energy
166
absorption spectra
where certain frequencies of light are missing because they're being absorbed by that element
167
Threshold frequency
the lowest frequency of light at which electrons are still released from a surface
168
what experiment determines the work function of different materials and the value of h?
stopping voltage experiment
169
What does the graph from the stopping voltage experiment show?
gradient = h
F0 (x intercept) = threshold frequency
y intercept = work function
170
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)
171
What axises are plotted from the stopping voltage experiment?
```
y = stopping voltage
x = frequency
```
172
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
173
if the pd. in a stopping voltage experiment is decreased what happens?
the battery is more effective than the photoelctric effect therefore the electrons are slowed and start to move backwards.
174
what is stopping voltage?
the voltage at which the battery becomes more powerful than the photoelectric effect and the electrons are slowed
175
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 electron.
176
Line spectra
Specific frequencies/wavelengths show the absorbtion/ emmision lines within a narrow line of wavelengths
177
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
178
wave model features
- diffraction
- refraction
- reflection
- have a frequency
- interfere with each other
- pass through each other
179
photon model features (features of particles)
- have mass
- reflect
- experiences forces between each other
- have volume
- can have charge
- have momentum
- have density
180
The shorter the pulse...
the shorter the distance that can be measured
181
why does the photon model work for 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 t the electron as kinetic energy
182
Why does the wave model not work for the photoelectric effect
- frequency would build up to high enough to liberate and electron
- K.E. would depend on the intensity of the light
183
long wavelength photon means...
less energy levels moved up
184
high frequency photon means...
the more energy levels it jumps up
185
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
186
how can electrons be excited?
- if a photon is absorbed
| - if electrons are hit be other electrons
187
energy delivered by photon (hf) =
difference between the energy levels
188
ground state
the lowest energy level where electrons are usually found
189
Why are only certain frequencies absorbed by atoms?
electrons can only exist in discrete energy levels
190
Kinetic energy gained by accelerating electron through a potential difference =
eV
191
the amount of diffraction that a wave undergoes depends on the
amplitude of the incident wave and the size of the opening
192
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)
193
why is diffraction move obvious with sound than light
sound has a longer wavelength so it occurs more at our scale
194
intensity of light through two Polaroids is greatest when
the Polaroids are parallel
195
hf
energy of a photon
196
Ф
The energy required from a single photon to release an electron from its energy well (work function)
197
kinetic energy of photoelectrons depends on...
the frequency of the incident photon
198
more intense light means....
more photoelectrons released (IF FREQUENCY OVER THRESHOLD FREQUENCY)
199
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)
200
Why are certain frequencies missing from an absorption spectrum?
- electrons get excited by absorbing photons
- electrons have fixed energy levels. only certain transitions possible, so only certain photon energies absorbed so some frequencies missing
- the set of frequencies absorbed depends on the element
201
energy of photon absorbed =
difference in energy levels
202
range of visible light wavelengths
400nm - 700nm
203
how to get the first order maxima closer together?
increase the frequency of the laser
204
as speed decreases...
wavelength decreases
205
wave property which only applies to transverse waves?
polarisation
206
not polarised
oscillates in all the planes perpendicular to the direction of travel
207
standing wave
a series of nodes and antinodes formed for interfering coherent waves
208
out of phase value in radians
pi
209
in phase value
0, 2 pi
210
fundamental frequency
lowest frequency of a standing wave that can be set
211
frequency of ultrasound
20 000 Hz
212
image from a lens where the object is beyond 2x the focal length
inverted
diminished
real
213
object on focal point convex lens
ray are parallel, no image will be formed
214
long sight
the power is too weak so it doesn't converge on the retina
215
short sight
to powerful, the image converges before the retina
216
milikans experiment
- they let the small drops of oil fall between the two plates
- by adjusting the pd 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
217
milikans 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
218
milikans experiment set up
electric field
atomiser to spray oil drops into the electric field
a microscope to view the oil drops
219
how did the find out r in milikans experiment
- let it fall at terminal velocity and then used forces
| - 6πνrv = mg = 4/3ρgπr^3
220
frequency from number of oscillations in a given time
f = number of oscillations / time
221
line spectra
specific narrow band of frequencies or wavelengths that an element has absorbed/emitted
222
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)
223
What does vertically polarised mean?
light only oscillates in the vertical plane perpendicular to the direction of travel
224
Why can two oppositely polarised sources not interfere
the oscillate perpendicular to each other so the opposite/ same components cannot undergo superposition
225
explain how refraction is caused (2 parts)
- materials are different densities
| - light changes direction and appears to come from another point
226
critical angle
the angle of incidence for light travelling from a denser medium has angle of refraction of 90
