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
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
if the intensity of unpolarised light is X, what is the intensity of this light after passing through one polarising filter?
Malus’ Law
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
what is polarisation
using a polariser, only light waves with oscillations in a certain direction are filtered through
Malus’ Law
gives the intensity of light after passing through a polariser
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
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
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
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
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.
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)
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)
When the frequency of light increases, the magnitude of the stopping voltage also increases.
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)
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
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
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
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)
explain why the force required to accelerate an electron changes as the electron’s speed increases
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
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
