M7 The Nature of Light Flashcards
regardless of wavelength, all EM waves travel at…
the speed of light
c
c is the speed an EM waves travels in a…
vacuum (although air resistance is negligible)
EM waves are longitudinal or transverse?
do EM waves need matter in order to propagate?
what is a wave?
the movement of energy without the movement of matter
EM waves do carry energy!
EM spectrum is made up of
7 principle radiations
7 principle EM radiations acronym
RMIVUXG
Ronald, may I visit ur x girlfriend?
Visible light spectrum wavelength
700nm - 350nm
visible light colours acronym
gamma ray facts
EM spectrum
high frequency (low wavelength) means
high energy
EM spectrum table
wave, wavelength (not really), example of application
Infrared: incandescent lamps, heat lamps
UV: lasers, sterilisation killing bacteria, dentistry
Gamma: cosmic rays, radioactive isotopes
exposure to EM radiation is harmful T or F
unification and existence of EM waves
- a changing magnetic field will produce a perpendicular changing electric field, and vice versa
- transfer of EM energy that is self propagating
Faraday’s law predicts that a changing magnetic field will produce…
a perpendicular changing electric field! A changing magnetic field through a region of space will produce an electric field across the same region of space perpendicular to the original magnetic field
According to Ampere’s law, a changing electric field will produce
Maxwell unified the theory of
An electric field will always be produced by a magnetic field
T or F
EM waves are produced by…
what does a charge acceleration produce?
in radio communication, sound waves are recorded and first converted into…
light is often emitted by atoms when…
what is spectroscopy?
explanation of absorption spectrum
the black spectral lines correspond with the wavelengths of EM radiation that have been absorbed by electrons
explanation of emission spectrum
the colourful spectral lines are the wavelengths of emitted EM radiation
(as materials will emit EM radiation to lose energy)
what is a spectrum of light
when an electron moves from a higher orbit to a lower orbit, it releases energy as
light energy (that’s how we get emission spectra)
the wavelength and energy of an EM wave are directly proportional
T or F
Which objects emit EM radiation?
When we observe the spectra of stars we are just observing the star’s emission spectrum
T or F
An alternating high voltage spark gap can be used to produce EM waves. How can we alter the type of EM wave produced?
The wavelengths an electron can absorb…
The spectrum emitted by a white torch is recorded. The torch is then shone straight through a tube of cooled hydrogen gas. The torch is viewed from the other side of the tube, and the spectrum observed is also recorded.
What changes should we expect in the second spectrum?
how are the LINES in the emission spectrum and the GAPS in the absorption spectrum of an object related?
they occur at the same wavelengths
lines at the emitted wavelengths
gaps at the absorbed wavelengths
when an atom emits EM radiation
when an atom absorbs EM radiation
emits: an electron moves down an energy level
absorbs: an electron moves up an energy level
The amount of energy an atom has to absorb to move up an energy level is…
That’s why…?
the same as the amount of energy it must release to move down an energy level
That’s why the emitted wavelengths are identical to the absorbed wavelengths
Atoms can absorb the energy from light to…
move electrons to more excited states!
Dense gases absorb…
and why?
more EM radiation
when a wave passes through a relatively dense gas, it is more likely to collide with an electron and be absorbed
Physical appearances of spectral lines for stars
- broader, less defined lines (element spectra) : denser gas
- red shifted (star spectra) : star is moving away from us
- blue shifted (star spectra) : star is moving towards us
- broader, less defined, stretched out both sides (star spectra) : star is rotating
density of the gas changes spectral lines how?
IT IS STILL THE SAME GAS
note the dense hydrogen and less dense hydrogen
Spectra of Stars
by matching the lines in the emission spectrum of different chemicals to…
the gaps in the absorption spectrum of a star, we can deduce the star’s chemical composition
hydrogen emission spectrum
4 lines
violet
indigo
light blue
orange
(rough)
translational velocity:
velocity of a celestial body relative to Earth
(translational velocity) if it’s moving away from Earth, the wavelengths in a star’s absorption spectrum will be
red shifted
the Doppler effect helps to determine the (stars)
translational velocity of a star
redshift: as the star moves away, the light waves are more stretched
blueshift: as the star moves toward, the light wavelengths are compressed
difference between cosmological redshift and Doppler redshift
The expansion of space stretches the wavelengths of light that is travelling through it.
Doppler redshift is from motion through space, while cosmological redshift is from the expansion of space itself
rotational velocity and explanation of effect on star spectra
how fast the star is spinning
when a star is rotating, one side is receding from us while the other side is approaching us. light emitted from the receding side will be red-shifted, light from approaching side will be blue-shifted. light from other parts of the star will fall within these 2 limits. this causes the spectral line to be broadened.
the faster the star is rotating, (spectra)
The Doppler effect can be used to determine the rotational velocity of a star. Explain
A rotating star has one side turning towards Earth and one side turning away from Earth.
The light coming from the approaching side will be blueshifted (since it’s rotating towards us, has a shorter wavelength)
The light coming from the receding side will be redshifted (since it’s rotating away, has a longer wavelength)
The shift can help determine the difference in speed between the two sides, which helps determine the rotational velocity.
If a star travels away from Earth, the light coming from it will be…
redshifted.
When sources and observers move away from each other, the Doppler Effect states the wavelengths will increase
As a star rotates, its observed absorption spectrum…
stretches out
Since light from one side is redshifted, and the other is blueshifted, the shift in both directions causes the spectrum to stretch outwards
The direction a star rotates will affect the spectra observed coming from the star
T or F
False!
incandescent filaments produce light by…
heating up to high temperatures, causing the filament’s electrons to gain energy and jump between energy levels which produces a EM radiation
Discharge tubes produce light through
fluorescence: a type of emission occurring when an electron absorbs EM radiation of a certain wavelength and emits it at a longer wavelength
a discontinuous of specific wavelengths in the visible spectrum are produced
spectra of discharge tubes, reflected sunlight and incandescent filaments
fluorescent coating of discharge tube only emits
visible light
which is more efficient and why? discharge tubes or light globes
we don’t need high temperatures to produce visible light so discharge tubes are more efficient
explain how light is produced in discharge tubes
When a voltage is applied to the electrodes of a gas discharge tube, an electric field is created between the electrodes. In that electric field, a free electron will accelerate toward the positive electrode. When such an electron collides with a gas molecule in its path, it may transfer some of its energy to the gas molecule, producing a gas molecule in an excited (high-energy) state. The energy emitted by the excited molecule is in the form of light.
The light emitted results from collisions between atoms in the gas and electrons of the current.
list the three components in a fluorescent tube and what they do
mercury vapour
voltage source
phosphorous coating
(NOT; UV light source)
The chemical composition of the star is what’s responsible for the absorption lines. Why?
Because absorption lines depend on the electron transitions in specific atoms in the star’s outer atmosphere
difference between a particle and a wave
A particle transfers matter and energy.
A wave transfers energy without a transfer of matter.
what is the permittivity of free space
How could EM waves be produced by high-voltage power lines?
High-voltage power lines contain electrons moving backwards and forwards in an alternating current. These electrons, which move with a frequency of 50 to 60 Hz, will generate EM waves with a frequency of 50-60 Hz.
Does a constant DC current produce EM waves?
No. Changing electric fields produces changing magnetic fields. Constant magnetic and electric fields will not produce an EM wave.
Compare the spectra of light from fluorescent and incandescent lights.
A scientific theory shows the consistency between different observations and experimental results. What are some of the discrete phenomena linked by Maxwell’s theory of electromagnetism?
just a fun fact
difference between the spectra produced by gas discharge tubes and incandescent lamps
in the 1860s, James Maxwell produced his theory of…
electromagnetism which encapsulated what was known about electricity and magnetism
EM waves are produced by accelerating charges for example
electrons oscillating at high frequency in a radio antenna produce radio waves
creation of visible light in a fluorescent tube and clarification
In a fluorescent tube, the initial high voltage gives the free electrons in the fluorescent tube energy to ionise (charge by removing electrons) the mercury atoms.
The initial voltage through the mercury doesn’t directly excite the electrons in mercury.
absorption spectrum is the spectrum of…
electromagnetic radiation passing through a substance, showing dark lines due to absorption at specific wavelengths
analyser is
A polarising device that shows the direction of vibration of light by
selecting and transmitting only the component of linearly polarised
light in that direction.
a theoretical black body is
an object that totally absorbs all EM radiation that falls on it, thus it does not REFLECT any light so appears black. as it absorbs energy it heats up and re-radiates the energy as EM radiation
crossed polariser
two ideal polarisers placed at right anlges to each other will stop any light being transmitted
emission spectra…
The spectra produced as electrons fall from a higher energy state to a
lower energy state resulting in a series of coloured bright lines
Kelvin temperature
A temperature scale having an absolute zero below which temperatures
do not exist. 0 K corresponds to a temperature of - 273.15 °C
photoelectron
an electron emitted from an atom due to the interaction of a photon
photon
a particle of EM energy (eg light)
polariser
An optical filter that lets light waves of a specific polarisation pass through while blocking light waves of other polarisations
polarised light
light waves where the vibrations occur in a single plane
threshold frequency f₀
the minimum frequency of light which causes emission of electrons
ultraviolet cataStrOPHIe
Classical theory predicted that as wavelength of radiation decreased, the radiation intensity would increase, without limit. This would mean that, as the energy (that was emitted from the black body and reabsorbed) decreased in wavelength, the intensity of radiation would approach infinity. This would violate the conservation of energy.
9 relevant constants
11 relevant formulae
- the model shows an oscillating charge producing an EM wave, which is consistent with Maxwell’s prediction that oscillating charges produce EM waves
- the model shows that the EM waves contain perpendicular electric and magnetic fields in waves, consistent with Maxwell’s prediction that changing electric fields produce perpendicular changing magnetic fields, which produce changing electric fields, and so on
- the model shows that both the electric and magnetic fields are perpendicular to the direction of propagation, consistent with Maxwell’s predictions that electric fields, magnetic fields and the propagation direction of EM waves will be perpendicular
- the model shows that the wave propagates a velocity of v, consistent with Maxwell’s prediction that EM waves of all wavelengths travel at the same speed v
the peak wavelength of an object’s emitted radiation is…
inversely proportional to temperature. High tempeature objects have more energy, so they emit higher energy EM radiation. Remember that waves with higher energy have shorter wavelengths. So, higher energy bodies will have smaller peak wavelengths
explain what the spectrum of each star, Croesus and Dromus, tells us about the motion of that star
the surface temperatures of stars can be determined by
using Wein’s law
wavelength of peak intensity is peak wavelength
CALCULATE IN KELVIN
translational velocity as broken up into vector components
to produce an EM wave,
(and calculate c)
an oscillating (vibrating) electric charge is required
the moving charge forms an electric current that is oscillating in magnitude and direction. this will induce a magnetic field that oscillates in magnetic and direction in proportion with the current. this changing magnetic field induces an electric field whose magnitude varies in proportion with the magnitude of the electric field.
these oscillating fields are self-perpetuating and radiate through space at c
Wein’s Law tells us that
objects of different temperature emit spectra that peak at different wavelengths. hotter objects emit most of their radiation at shorter wavelengths hence they appear bluer
stars with higher density have…
(wavelength vs intensity graph)
broader and sharper intensity drop offs at their absorption lines because of the greater chance of radiation absorption
wavelengths with high relative intensity correspond to
wavelengths with low relative intensity corresponds to
wavelengths with high relative intensity spike in intensity around…
emitted wavelengths
what is the source of the spectrum and what feature is labelled Y?
source: star
Y: absorption lines
the overall shape of the spectrum is most similar to a black body radiation curve, which means that the spectrum is of a star and not a gas discharge tube. then, the drops in intensity represent absorption lines as emission lines would increase the intensity.
Wien’s Law eq
what are the axes of a blackbody radiation graph
x axis: wavelength
y axis: intensity
it shows the wavelengths emitted from the blackbody
we would expect a hotter blackbody to emit more radiation at the blue end of the visible spectrum rather than the red end. t or f?
true
a blackbody’s peak wavelength is inversely proportional to temperature. this means the hotter it gets, the more short wavelength light it emits
Young’s Double Slit Experiment
light must be coherent and monochromatic
coherence is necessary because we need the wavelengths to be consistent to produce interference patterns
supported the wave mode of light
demonstrated two source interference with light, resulting it diffraction patterns
bright and dark fringes
2 sources interfere constructively or destructively
con: occurs when 2 waves are in phase. a resultant wave will be produced with an amplitude that is the sum of the 2 individual waves. gives rise to the bright spots
des: occurs when 2 waves are out of phase. the 2 waves will cancel eachother out, giving rise to the dark spots
path difference visual
double slit wave interference visual
dsinθ = mλ
(& x)
what condition is required to use dsinθ = mλ?
the distance between the slits and the screen should be much larger than the slit separation
how do polarised sunglasses reduce glare reflecting off bodies of water?
they block horizontally polarised light but are transparent to vertically polarised light. this is because horizontal surfaces reflect horizontally polarised light, which is the main cause of glare
if the intensity of unpolarised light is X, what is the intensity of this light after passing through one polarising filter?
Malus’ Law
Huygen proposed that light was a longitudinal wave but
in the early 19th century, physicists became aware of polarisation, a phenomenon that can only occur with transverse waves
diffraction in simple terms
the spreading of waves as they pass through or around an obstacle
why does a path difference of nλ result in constructive interference?
(n is an integer)
a path difference that is an integer number of wavelengths will ensure that the waves stay lined up; crests stay atop crests, and troughs stay on top of troughs
and draw a conclusion that can be made regarding the nature of light based on this experiment
as the light wave passes by the edges of the spherical object, the wave is diffracted and bends arounds the spherical object.
this results in the interference patterns observed.
the bright dot in the middle indicates that all waves reaching this point are in phase, and there will always be constructive interference at this spot. this happens because the path difference is 0 and the waves are travelling the same distance to reach this spot
conclusion: from this experiment, it is shown that light has wave properties of diffraction and interference
constructive and destructive wave interference visual
number of lines per metre (diffraction grating)
if a diffraction grating has Y lines per metre then
d = 1/Ym
eg
A grating has 5000 lines per metre
Hence d = 1/5000 = 2 x 10^-4 m
Huygen’s theory requires that the speed of light in air should slow down when
entering a denser medium
Newton said it would speed up
what is unpolarised light
made up of many light waves with each of their electric field vectors in different directions
what is polarisation
using a polariser, only light waves with oscillations in a certain direction are filtered through
polarisation is a property of…
transverse waves
it is the restricting of the plane of oscillation of a transverse wave to just one direction
Malus’ Law
gives the intensity of light after passing through a polariser
what is always true about 2 parallel beams of light with the same polarisation?
always have electric fields that oscillate in same direction
how does the wave model and particle model of light explain refraction?
wave: Refraction occcurs because the light slows down when entering a denser medium (bend toward normal) (wavefront to change direction)
particle: particles experience an attractive force that increases the normal component of particle’s velocity but not tangential (effect: bending towards the normal), travelling faster in a denser medium
Huygen’s wavelet visual
monochromatic light
light that has the same wavelength, only one colour
2 beams of light are coherent when
the phase difference between their waves is constant
if the difference between the path lengths is a multiple of
whole wavelengths
half wavelengths
waves will arrive in phase
waves will arrive out of phase
with light, we cannot tell the difference between a trough and a crest…
we either see a bright spot with constructive interference or dark with destructive interference
Explain how any one of the dark bands forms on the screen
The distance between the centres of the double slit is 0.15mm, and the distance between the double slit and the screen is 0.75m. Calculate the distance on the screen from the centre of the central maximum to the centre of a second-order bright band
compare wave theory and corpuscular theory (INCLUDE YT NOTES)
when graphing Malus’ Law
polarised light has maximum intensity at 0 and 180 degrees, and minimum values at 90 and 270.
x-axis: θ = cosine function
x-axis: cos²θ = straight line
particle vs wave
how light travels
Newton: once ejected from a light source, the particles continue in a straight line until they hit a surface
Huygen’s Principle: every point on a wavefront may be considered to act as a source of circular secondary wavelets that travel in the direction of the wave
the more slits on a diffraction grating
the sharper the image obtained
diffraction grating visual
particle vs wave
reflection of light
Newton: as particle approaches a surface, they are repelled by a force that slows down and reverses the normal component of velocity (angle of incidence = angle of reflection)
Huygen: as each part of the wavefront approaches surface, it produces a reflected wavelet
get quick notes on how the speed of light was measured
diffraction and interference patterns with white light
evidence against the wave model
black body radiation
photoelectric effect
what is a photon
a quantised “packet” of light energy
equation for energy of a photon
Explain how the result of one investigation of the photoelectric effect changed the scientific understand of the nature of light
if you steadily decrease the frequency of the incident EM radiation in the photoelectric effect, eventually no spark will be observed
T or F
True, this is the threshold frequency
At any frequency below the threshold frequency, photons do not have enough energy to eject electrons from the material’s surface
what is the photoelectric effect
the emission of electrons from the surface of a conductor when subject to EM radiation
if a photon collides with an electron, all of its energy is transferred
only one photon can collide with each electron
what is a photoelectron
an electron that has absorbed a photon and been emitted
Only certain types of EM radiation can cause the photoelectric effect
t or f
any type of light can cause photoelectron emission so long as the energy of the photon is greater than the energy holding the electron to its orbit
Photoelectron emission involves an electron being excited out of its atom’s electron shells
True
what happens when a photon collides with an electron that has a larger orbital energy than the photon’s energy
the photon transfers all its energy to the electron, but causes no emission
(photoelectric effect)
describe the energy of electrons before and after emission
Before emission, the electrons have negative energy because they are bound to the surface of the conductor. An emitted electron has gained enough energy to escape the surface, so its kinetic energy after emission is positive
what has the greatest effect on the work function
it is specific to the surface conductor material
A voltmeter would be able to detect the impact of higher frequency radiation on a photocurrent! The higher frequency means each photoelectron will carry more energy. Higher energy per electron means a higher voltage.
4 Con/InCon sistencies with the wave and particle model (photoelectric effect)
W: wave intensity (amplitude) is proportional to energy and increasing light intensity would increase photoelectron energy
P: changing the intensity has no effect on the energy of the photoelectrons (however, photocurrent depends on intensity)
WW: wave frequency and electron energy are unrelated
but rate of electron emission should increase with frequency
PP: changing frequency of incident light changed the max kinetic energy of photoelectrons (below a certain frequency, no electrons were emitted) (photocurrent/rate of electron emission didn’t increase with frequency)
WWW: delay between light being turned on and electron emission (since light “waves were thought to be continuous”) (the oscillating electric field of the incident light wave was heating the electrons, causing them to vibrate, and eventually eject from metal’s surface), there was predicted to be a noticeable time lag between the light wave transferring its energy to the surface’s electrons)
PPP: instantaneous
WWWW: no threshold frequency/intensity/effect should exist as energy transfer from light to electrons is accumulative and emission will occur eventually
PPPP: threshold frequency is predicted, as photons with energy less than the work functions can’t free electrons
difference between incident photon energy and photoelectron energy
incident photon energy is calculated using E = hf
photoelectron energy is the max kinetic energy after emission
In a particular photoelectric effect experiment, scientists found that a negative applied voltage decreased the photocurrent whereas a positive applied voltage increased the photocurrent
How does an applied voltage increase photocurrent?
by increasing the potential difference across electrodes, allowing for photoelectrons to travel from one electrode to the other (electromotive force)
Increasing the intensity of light used in a photoelectric effect experiment will…
increase the measured photocurrent
the measured photocurrent is based on the number of photons in the light source
photocurrent only flows when there is an applied positive voltage
t or f
false
applied pos voltage will increase, but there can still be photocurrent with no voltage or neg voltage
the photocurrent increases without limit as the applied voltage increase
t or f
false
the photocurrent will increase until it plateaus at a maximum value. the intensity of light source determines the maximum photocurrent because it sets the number of photons that are hitting the metal
Theory and Formulae (photoelectric effect)
Threshold Frequency (f₀)
the minimum frequency of light that can induce photoelectric effect
Work Function (Φ)
The lowest amount of energy an electron needs to escape an atom
Maximum Kinetic Energy (Eₖ/KEₘₐₓ)
The largest amount of kinetic energy the electrons can gain from a given frequency of light
Photocurrent
The current that flows through a circuit due to the photoelectric effect (emitted photoelectrons in a circuit). It is proportional to the intensity of light as more photons means more photoelectrons.
Stopping Voltage (V₀)
The value of applied voltage when there is no current. This “photocurrent” is the current produced by the electron being ejected from the emitter plate and travelling to the collector. It is just enough to prevent the emission of photoelectrons. Looking at diagram, initially an applied voltage (clockwise) renders the collector plate negative so the maximum kinetic energy can be ascertained. Then, a backing/stopping voltage is applied that moves anticlockwise so as to apply a force against the electron flow (clockwise). When the ammeter reads 0A, you have the stopping voltage.
hf = Φ + KEₘₐₓ
Φ = hf₀
The ENERGY the stopping voltage applies is equal to KEₘₐₓ in eV
qₑV₀ = max KE (not hf as that is the energy of the photon)
V₀ = KE/qₑ (KE in J)
Graphs (photoelectric effect) 6
x: applied voltage
y: photocurrent
(increasing frequency/increasing intensity)
x: frequency
y: photoelectron max KE
x: intensity of light
y: photocurrent
x: stopping voltage
y: photocurrent
(increasing frequency/increasing intensity)
x: intensity of light
y: KEmax
x: frequency
y: photocurrent
Celsius & Kelvin conversion
C → K
add 273
(alphabetical order, add!)
K → C
subtract 273
eV & J conversion
eV → J
x 1.6 x 10⁻¹⁹ (1 eV = very little J, so we multiply by small number)
J → eV
÷ 1.6 x 10⁻¹⁹
(a joule is too large a unit to use with photons)
why does the photocurrent reach a maximum value as the applied voltage is increased for a given intensity of light?
all available photons are being used to produce the photocurrent. the intensity of light sets a limit on the total photocurrent based on the number of photons striking the metal
The stopping voltage depends on
the type of metal used. The stopping voltage is the voltage required to fully stop the photocurrent which depends on frequency of light and work function of the metal
When the frequency of light increases, the magnitude of the stopping voltage also increases.
the photocurrent at zero applied voltage is independent of frequency and only depends on the metal used
t or f
false
higher frequencies of light will create a higher photocurrent as the energy of the photon can overcome the work function and increase max KE so more photoelectron emission AS LONG AS the maximum photocurrent (max number of photoelectrons so intensity of light) hasn’t been reached
if the max photocurrent has been reached, changing the frequency or metal will only change the max KE of the photoelectrons
explain how Planck accounted for the discrepancy between the experimental and theoretical black body radiation curves
Planck proposed that EM energy wasn’t emitted and absorbed continuously, but in discrete packets called quanta.
The discrete amount of energy released and absorbed to transition between energy levels (light released) is the energy of the photon (E=hf).
TO EXPLAIN THE DROP OFF IN INTENSITY (suggested by Planck) energy transitions around peak wavelength were more likely than transitions at low wavelengths (high frequencies)
explain how the failure of high wavelength EM radiation to cause the emission of electrons from materials demonstrates the particle nature of light
Classical model of light proposed that proposed that light of any frequency would cause electron emission as long as the intensity was great enough, as the electron would eventually absorb the continuous energy of the wave to emit.
the emission of electrons can instead be explained by the particle model of light as each photon as a discrete amount of energy dependent on the light frequency. this means once photons transfer their energy to the electrons, the electrons can instantaneously eject.
at large distances from a point source of light, the intensity…
is low because the photons are spread over a large area, bu there is no loss in energy of each photon
does the stopping voltage change with incident EM radiation intensity
NO! since stopping voltage is proportional to the max KE of photoelectron, since radiation intensity doesn’t change KE, it also doesn’t change stopping voltage
Stopping V vs Photocurrent graph
increasing light frequency
for a given light intensity (so the same photocurrent), increasing frequency of light increases the max KE of the photoelectrons, increasing the stopping voltage
Stopping V vs Photocurrent graph
increasing light intensity
for a given frequency, photoelectrons are emitted with the same KEmax so have the same stopping voltage but have different photocurrents depending on the intensity
frequency vs KEmax (graph)
x intercept is threshold frequency
y intercept is work function
gradient is Planck’s constant h
applied voltage vs photocurrent graph
increasing intensity
increasing intensity increases the number of photoelectrons, so the maximum photocurrent increases
curve flattens at max photocurrent
stopping voltage (x intercept) stays the same
applied voltage vs photocurrent graph
increasing frequency
increasing frequency doesn’t change max photocurrent but changes stopping voltage (x axis)
applied voltage vs photocurrent graph
intensity vs photocurrent (graph)
as long as frequency is above threshold frequency
intensity vs KEmax graph
frequency vs photocurrent graph
assuming the frequency of incident light is greater than the threshold
1.82eV
calcium
work function for metal:
Φ = hf₀ = 3.65 x 10⁻¹⁹ J
energy difference:
E = hf₆ₗᵤₑ - Φ
= 4.94 x 10⁻²⁰ J
work done by voltage source (energy difference) = 4.94 x 10⁻²⁰ J
V₀ = W/qₑ
0.31 V (stopping voltage)
blue, yellow-white, red stars approximate temp
blue: 10000K
yellow-white: 6000K
red: 3000K
Timeline of key discoveries about the photoelectric effect (4 events)
1887: Hertz noted unusual behaviour of sparks across the gaps in his radio wave detector circuit
1901: Planck solves black body radiation problem theoretically, modelling that light is not just a wave
1902: experiments are carried out, and the electron energy didn’t depend on light intensity, and unique cut-off frequency for each material
1905: Einstein used particle model of light to predict the graph of stopping voltage vs frequency, would be straight, slope is same for all metals
A spectroscope…
splits light up so that the different components are spread out allowing the intensity of light at different wavelengths to be observed
(black body curves)
how to manipulate to get a straight line graph
wavelength on x axis
inverse of temperature on y axis
gradient of graph is Wien’s constant
relevant prefixes
Mega (M) +6
kilo (k) +3
milli (m) -3
micro (μ) -6
nano (n) -9
another huygen wavelet visual
Measuring c
Romer
1670s using the eclipse of Io (Jupiter moon)
Further away from Jupiter (F,K), Romer measured a greater period for Io than when Earth was closer to Jupiter (L,G). He inferred that light had a finite speed.
(not including when Sun and Jupiter obscure Io (K to F minor arc)
Measuring c
Fizeau
1849 using a half-silvered mirror and a toothed wheel
Maxwell’s 4 Equations
Guass’s Law: electric charge acts as sources for Electric Fields
Guass’s Law: the net magnetic flux out of any closed surface is zero, and no such thing as a magnetic monopole
Faraday’s Law: make an electric field by changing a magnetic field
Ampere’s Law: make a magnetic field with a changing electric field or with a current
Newton’s explanation of dispersion
white light was made of all the colours
red corpuscles were bigger than blue corpuscles so the prism’s particles had less effect on them so red light was least refracted
the fine line of intensity, energy, number of photons
Red light carries less energy per photon due to its lower frequency.
Blue light carries more energy per photon due to its higher frequency.
If total energy emission is equal, red light compensates for its lower energy photons by emitting a greater number of them. Blue light requires fewer photons to deliver the same total energy.
Chocolate Light Experiment
Electrical circuits inside your microwave appliance generate microwaves, EM waves with frequencies around 2.5 gigahertz (10⁹). These waves bounce back and forth between the walls of the microwave. Peaks and troughs of the reflected waves superimpose with other microwaves to form a standing wave. The antinode produces the most heating, and the nodes the least.
Method:
Remove the turntable from the microwave and cover the rotating gear with a bowl. Place a plate of a bar of chocolate on top. Heat the chocolate. There will be some spots with the most melting. The distance between 2 adjacent “hot spots”/antinode is half the wavelength of the microwave. Use equation c = fλ
inertial frame of reference, is there an experiment to discern between constant velocity and stationary.
no but you can discern whether you in non-inertial or inertial frame of reference
besides pendulum, you could roll a ball on the table, throwing a ball in air
what are the features of the principle of relativity?
there is no such thing as universal rest, which is the same thing as saying there is no universal reference frame. it also says there is no experiment that can be performed to distinguish between being stationary and moving a constant velocity
any experiment in one inertial frame will go the same as in another inertial frame
2 postulates of special relativity
- all inertial frames of reference are equivalent (the principle of relativity holds in all inertial frames)
- the speed of light is constant in all inertial frames (in a vacuum)
time is measured to be the same in every inertial reference frame. t or f
false
a consequence of special relativity is that time is measured to be different depending on which reference frame you are in
the relativity of simultaneity is when
two events appear simultaneous for one observer but not to another if they are in relative motion
both observers are correct for their respective reference frames
what is the reason for the relativity of simultaneity?
RoS is a direct consequence of the constancy of the speed of light.
If light had a relative speed, distance and time would not need to be variable (constant) in different reference frames, and simultaneous events would be observed consistently
time dilation (theory)
time appears to move slower for people travelling closer to the speed of light
time stretches as we go faster
*** according to you, you measure your time like you always did. an OBSERVER WATCHING YOU would see your heart beat run slow
SINCE YOU ARE IN AN INERTIAL FOR, TO YOU, YOU ARE STILL
length contraction (theory)
the length of an object moving at a relativistic speed appears smaller to an observer WATCHING it move
only contracts in direction of movement
(parallel to the relative velocity between the two reference frames)
*** you measure your length as your always did, it is an outside observing who sees length contraction
SINCE YOU ARE IN AN INERTIAL FOR, TO YOU, YOU ARE STILL
in Einstein’s time dilation thought experiment, both observers will state that the other person’s time appeared to move…
both observers are “stationary” and see the other moving at a high velocity, meaning both will say the other’s time was dilated
length is observed to be contracted for…
(observed) moving reference frames
in your own reference frame, length will be
measured the same
energy is produced by
nuclear fusion
antimatter annihilation (colliding particles release energy)
combustion
explain why the force required to accelerate an electron changes as the electron’s speed increases
how can paradoxes that arise from the implications of time dilation and length contraction be explained?
recognise that two different reference frames are involved
explain the significance of the principle of relativity
examples of time dilation
twins paradox
We all go on a journey into the future; we cannot stop time. Relativity shows us that the rate that time progresses depends on the movements we make through space on the journey. Coasting along in an inertial frame of reference is the longest path to take. Zipping through different reference frames then returning home enables objects to reach the future in a shorter time; they take a longer journey through space but a shorter journey through time.
parking spot paradox
time dilation (formula)
tₔ = time observed by stationary observer, DILATED time
t₀ = time observed by an observer who is moving with the object (proper time: the time measured in the frame of reference where 2 events happen at the same place)
v = velocity of moving object
c = speed of light
output is bigger than input
the photon carries an amount of energy that is equivalent to an amount of mass but
the photon itself does not have mass
Lorentz Factor gamma γ
Lorentz Factor is always greater than 1
length contraction (formula)
l₀ = length observed by an observer moving with object (proper length: length measured in the reference frame where the object is at rest)
l = length observed by stationary observer
v = velocity of moving object
c = speed of light
output is smaller than input
outline experimental evidence that validated the prediction of time dilation
Michelson - Morley experiment
there was thought to be a medium for light - the Aether
therefore there would be a relative velocity between Earth and Aether (Earth moving through Aether) and thus light was expected to take different amounts of time to travel in different directions through the aether as Earth moved through it.
Model was wrong. Speed of light is unaffected by the motion of the reference frame of its observer or its source!
Michelson - Morley experiment apparatus
Muons can be detected on the surface of the Earth because…
Observation of cosmic origin muons at Earth’s surface
muon diagram
A spaceship travels to a distant star at a constant speed v. When it arrives, 15 years have passed on Earth but 9.4 years have passed for the astronaut on the ship.
a) What is the distance to the star as measured by an observer on Earth?
Relativistic Momentum
