05 waves and the nature of light Flashcards
longitudinal waves
a type of wave in which the particles oscillate parallel to the direction of the wave
What is a wavelength?
the distance between two matching points in neighbouring waves
amplitude
the maximum displacement a point moves from its centre of oscillation
The larger the amplitude…
the greater the energy
period
the time taken for a point on a wave to move through one complete oscillation
frequency
the number of oscillations per second
frequency =
1/time period
describe electromagnetic waves
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.
higher frequency means…
more energy
what is the order of the electromagnetic spectrum?
radio ( longest wavelength, least freq)
Microwaves
Infared
Visible light
Ultra violet
X rays
Gamma (shortest wavelength, highest frequency)
what is the wavelength of a radio wave?
10^3 - 10^1 m
what is the wavelength of a microwave?
10^-2 m
what is the wavelength of a Infra-red wave?
10^-5 m
what is the wavelength of a visible light wave?
10^-7 m
what is the wavelength of a ultra violet wave?
10^-8 m
what is the wavelength of a x-ray wave?
10^-10 m
what is the wavelength of a gamma ray?
10^-12 m (+)
What is diffraction?
is the spreading out of a wave as it goes past an obstacle or through a gap
What is Huygens principle?
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
What is a diffraction grating?
an optical component with a periodic structure that splits and diffracts light into several beams travelling in different directions.
constructive interference
is known as in phase
They supervise each other where peaks aligns causing the amplitude to be both peak amplitudes adding to each other
What is superposition?
when the wave amplitudes are added
destructive interference
is known as out of phase
(In anti phase)
When one waves peak Aligns with another’s trough causing the amplitude to be 0
degrees to radians conversion
radians = (degrees * pi)/180
radians to degrees conversion
degrees = (radians * 180) / pi
For two waves of light to be coherent the waves must
originate from one source
how does an additional converging lens affect the eye
decreases the image distance as the lens adds more power
virtual image
cannot be projected onto a screen
unit of viscosity
PaS
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)
wave speed
the speed the wave travels. v = fλ
wave equation
v = fλ
wave
something that transfers energy from one point to another
describe a longitudinal wave
particles in metal vibrate along direction of propagation making compressions and rarefactions
frequency time period equation
f = 1/T
dispersion
when waves separate out due to a wave travelling through a different medium (different wavelengths travel at different speeds)
intensity equation
I = P/A
intensity distance relationship
inverse square law
intensity
I
speed of sound in air
340 m/s
speed of light in a vacuum
3e8m
phase difference
how much one wave is in front or behind another wave
Transverse wave
a type of wave in which particles oscillate perpendicular to the direction of energy transfer in the wave
rarefraction
regions of low pressure due to particles being spread further apart.
compression
regions of high pressure due to particles being close together
how does a graph show transverse waves
displacement distance graph
How does a graph show longitudinal waves
displacement time graph
what type of wave an EM wave
transverse waves
wavefront
the leading edge of one complete wave
coherence
same frequency + unchanging phase difference
superposition
The resultant displacement can be found by adding the two displacements together from interfering waves
when does superposition occur?
2 waves are in phase
superposition of coherent waves
provides a constant pattern of interference
path difference
the difference in distance traveled by the two waves from their respective sources to a given point on the pattern
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.
What is the structure of an EM wave?
electric and magnetic fields which oscillate in phase and are perpendicular to each other
progressive wave characteristics
transfers energy
standing/stationary wave characteristics
stores energy
how are stationary waves formed in a string?
the wave reflects back from a terminator and interferes with itself
resonant frequencies
a natural frequency of vibration determined by the physical parameters of the vibrating object.
harmonics
a wave where its frequency is a multiple of the material natural frequency resulting in a standing wave
Where are nodes on a standing wave in a string
at the end of the string (+in between depending on the harmonic)
mass per unit length
mass of an object divided by its length
how to calculate wave speed of a standing wave on a string
V = √(T/μ) where μ is the mass per unit length
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.
How is stationary waves in a pipe drawn in a diagram?
drawn as a displacement distance graph
where is a node formed in a closed pipe standing wave
at the closed end
why is a node formed at the closed end of a pipe
the air cannot oscillate freely
Where is an antinode formed in a closed pipe
at the open end
whats different about closed pipe harmonics
it can only form odd harmonics
Where is a anti node formed in an open pipe
at both ends (because they’re open)
wave diagram
shows the wave fronts (straight lines perpendicular to direction of travel)
ray diagram
shows a single ray and the direction and action of a wave
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
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.
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
refraction
the change in direction of wave propagation due to it moving through a different medium
why does refraction occur
waves travel at different speeds in different mediums.
less dense -> more dense
bends towords the normal as light slows down
snells law
n = sini/sinr = c/v
refractive index letter
n
absolute refractive index
a ratio of the speed of light in a vacuum to the speed of light in a given medium
1n2 =
calculating the refractive index between two materials
critical angle
the largest angle at which refraction out of a denser medium is possible
refraction between two mediums equation
n1sin θ1= n2 sin θ2
how do you calculate the critical angle?
by making θ2 90 degrees in the equation: n1sin θ1= n2 sin θ2
absolute refractive index of air
1
absolute refractive index of water (use to check calculations)
1.33
total internal reflection
the complete reflection of a wave where the angle of incidence exceeds the critical angle
if i is less than the critical angle then
refraction
if i = critical angle =>
partial TIR (multiple rays)
if i>critical angle =>
TIR
how to measure the refractive index of a solid material
use a glass block to shine light through and trace the path
focal length
the distance between the optical centre and the principle focus
diverging lens
a concave lens
converging lens
a bulging lens
convex lens
converging
concave lens
diverging
ray bending in a converging lens
going in: bends towards the normal
if the object is between the focal length and a converging lens then the image is
magnified
if the object is beyond the focal length of a converging lens then the image is
magnified
virtual principal focus
is you trace back the diverged rays to a single point
power of a diverging lens
always negative
What does diverging lens do to the image
diminished
what does the focal length depend on
the curvature of the surface and the material used
the more powerful the lens the…
shorter the focal length
power of a lens equation
P = 1/f
lens equation
1/f = 1/u + 1/v distances to real objects and images are positive
magnification equation
magnification = image distance/ object distance
real image
an image that can be projected onto a screen
virtual image
an image that can’t be projected onto a screen (appears to come from behind the lens)
combining lens powers
P = P1 + P2 + P3… for thin lenses
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
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
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
not polarised
wave oscillates in all directions
plane polarised
wave oscillates in one plane only
plane polarised examples
scattered/reflected light
polarising longitudinal waves
can’t be done
crosses polarised
when filters are perpendicular to each other so no light can get through
diffraction
the spreading out of a wave as it goes past an obstacle or through a gap
When a wave passes through a gap that is a similar size to their wavelength…
there is a lot of diffraction
monochromatic
only one wavelength
interference pattern
a series of maximum and minimum points that can be seen on a screen from interfering coherent waves
nλ =
dsinθ
d =
slit spacing
q
a
how to calculate d from lines per m
n = 1/(lines per m)
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
What is a Quanta?
Discrete packets of energy
Planck’s equation
E=hf
h value
6.633e-34
What did Einstein theorise?
That concentrated packets of energy had particle-like properties and were called photons
Photon
Concentrated discrete packets of energy which have particle-like properties
What is the EM spectrum from a particle point of view?
Many photons with different levels of energy
How much do photons weigh?
Weightless
How can photons travel at the speed of light?
Because they’re weightless
What letter represents the speed of light?
c
How is the equation E = hc/λ formed?
Combining E=hf and c=fλ
Electron volt
One electronvolt is the energy gained by an electron when it is accelerated through a p.d. of 1v (W= QV)
How to convert joules to eV
Divide by 1.6x10^-19
How to convert eV to Joules
Multiply by 1.6x10^-19
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 E = hc/λ, input gradient value and rearrange to get h.
Who worked out the photoelectric effect?
Einstein
What is the photoelectric effect?
The emission of electrons from the surface of, generally, a metal in response to incident light.
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.
How can the charge of an electroscope be found?
The angle the gold leaf lifts to
Why does the wave model not back up the photoelectric effect?
All the frequencies should combine energy to liberate the electrons
How many photons can liberate a single electron?
1
If wavelength increases…
Frequency decreases; therefore, electrons have less kinetic energy and eventually none are liberated.
If wavelength decreases
Frequency increases; therefore, electrons have more kinetic energy.
If intensity increases
More electrons are released, but with the same kinetic energy. If it is below the threshold frequency, intensity has NO effect.
Electrons are trapped inside __________ and in order to escape it has to ________
Energy wells; absorb enough energy
How does the material affect the energy well?
Different sizes, therefore different amounts of energy are needed to liberate the electrons
Work function
The amount of energy needed for the electrons to escape their energy well
Which formula works out the work function
hf = Φ + E.K. max
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
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
Photoelectron
A liberated electron
Intensity is proportional to
Rate of emission of photoelectrons
Broglie said that for
Any particle that had momentum, it also has wavelength λ = h/p
Relativistic mass
As a particle gets closer to the speed of light, the mass tends to increase due to relativistic effects
The intensity of a wave at a point represents
The probability of a wave being there
The electrons have _____ different energy levels but its energy is _______
Infinite; finite
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
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
Absorption spectra
Where certain frequencies of light are missing because they’re being absorbed by that element
Threshold frequency
The lowest frequency of light at which electrons are still released from a surface
What experiment determines the work function of different materials and the value of h?
Stopping voltage experiment
What does the graph from the stopping voltage experiment show?
Gradient = h; F0 (x-intercept) = threshold frequency; y-intercept = work function
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)
What axes are plotted from the stopping voltage experiment?
y = stopping voltage; x = frequency
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
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.
What is stopping voltage?
The voltage at which the battery becomes more powerful than the photoelectric effect and the electrons are slowed
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.
Line spectra
Specific frequencies/wavelengths show the absorption/emission lines within a narrow band of wavelengths
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
Wave model features
Diffraction, refraction, reflection, have a frequency, interfere with each other, pass through each other
Photon model features (features of particles)
Have mass, reflect, experience forces between each other, have volume, can have charge, have momentum, have density
The shorter the pulse…
The shorter the distance that can be measured
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.
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.
Long wavelength photon means…
Less energy levels moved up
High frequency photon means…
The more energy levels it jumps up
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
How can electrons be excited?
If a photon is absorbed; if electrons are hit by other electrons
Energy delivered by photon (hf) =
Difference between the energy levels
Ground state
The lowest energy level where electrons are usually found
Why are only certain frequencies absorbed by atoms?
Electrons can only exist in discrete energy levels
Kinetic energy gained by accelerating electron through a potential difference =
eV
The amount of diffraction that a wave undergoes depends on the
Amplitude of the incident wave and the size of the opening
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)
Why is diffraction more obvious with sound than light?
Sound has a longer wavelength so it occurs more at our scale
Intensity of light through two Polaroids is greatest when
The Polaroids are parallel
hf
Energy of a photon
Ф
The energy required from a single photon to release an electron from its energy well (work function)
Kinetic energy of photoelectrons depends on…
The frequency of the incident photon
More intense light means…
More photoelectrons released (IF FREQUENCY OVER THRESHOLD FREQUENCY)
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)
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.
Energy of photon absorbed =
Difference in energy levels
Range of visible light wavelengths
400nm - 700nm
How to get the first order maxima closer together?
Increase the frequency of the laser
As speed decreases…
Wavelength decreases
Wave property which only applies to transverse waves?
Polarisation
Not polarised
Oscillates in all the planes perpendicular to the direction of travel
Standing wave
A series of nodes and antinodes formed for interfering coherent waves
Out of phase value in radians
pi
In phase value
0, 2 pi
Fundamental frequency
Lowest frequency of a standing wave that can be set
Frequency of ultrasound
20,000 Hz
Image from a lens where the object is beyond 2x the focal length
Inverted, diminished, real
Object on focal point convex lens
Rays are parallel, no image will be formed
Long sight
The power is too weak so it doesn’t converge on the retina
Short sight
Too powerful, the image converges before the retina
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.
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
Millikan’s experiment set up
Electric field, atomiser to spray oil drops into the electric field, microscope to view the oil drops
How did they find r in Millikan’s experiment?
Let it fall at terminal velocity and then used forces; 6πνrv = mg = 4/3ρgπr^3
Frequency from number of oscillations in a given time
f = number of oscillations / time
Line spectra
Specific narrow band of frequencies or wavelengths that an element has absorbed/emitted
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)
What does vertically polarised mean?
Light only oscillates in the vertical plane perpendicular to the direction of travel
Why can two oppositely polarised sources not interfere?
They oscillate perpendicular to each other, so the opposite/same components cannot undergo superposition
Explain how refraction is caused (2 parts)
Materials are different densities; Light changes direction and appears to come from another point
Critical angle
The angle of incidence for light travelling from a denser medium that has an angle of refraction of 90
