Waves Flashcards
What is a progressive wave?
A wave that transfers energy from one place to another without transferring any matter. The transfer of energy is in the same direction as the wave is travelling.
State four ways you can demonstrate a progressive wave transfers energy.
EM waves cause things to heat up which transfers energy.
X-rays and gamma rays knock electrons out of their orbits, causing ionisation.
Loud sounds cause large oscillations in air particles which can make them vibrate.
Wave power can be used to generate electricity as waves can carry energy.
Define wavelength
The distance between two identical points on a wave.
Define amplitude.
The maximum displacement of a wave from its equilibrium position.
Define period.
The time taken for a wave to complete one full cycle.
Define frequency
The number of complete oscillations of a wave per second.
What is the wave speed equation in symbols?
v = f x λ
How is time period linked to frequency?
f = 1/T
What is a transverse wave?
A wave whose oscillations are perpendicular to the direction of energy transfer.
What is a longitudinal wave?
A wave whose oscillations are parallel to the direction of energy transfer.
Describe a longitudinal wave in terms of pressure variation and the displacement of molecules.
Longitudinal waves in a fluid consists of areas of compression and rarefaction. These compressions and rarefactions creature variations in pressure in the medium.
Areas of high pressure (compression) are where molecules in the medium are close together.
Areas of low pressure (rarefaction) are where molecules in the medium are far apart.
Areas of rarefaction move into vacant spaces creating vacant spaces behind them. Areas of compression push neighbouring particles into vacant spaces. This allows the movement of the wave.
What is a wavefront?
A line joining all points in a wave that are in phase and are drawn perpendicular to the direction of wave travel.
What is a standing wave?
A wave that stores energy at the nodes and antinodes instead of transferring it.
What conditions are needed to produce a stationary wave?
Two progressive waves must be travelling in opposite directions in the same medium, in the same plane.
The waves must be coherent; same frequency, same wavelength.
The two waves superpose together.
What is a node?
It is a point at which the displacement in a standing wave is at its minimum due to the wave being in antiphase, causing total destructive interference.
What is an antinode?
It is a point at which the displacement in a standing wave is at its maximum due to the wave being in phase, causing constructive interference.
Define superposition.
When the total displacement of a wave at any point is equal to the vector sum of the individual displacements.
Define coherence
Same type of waves that have same wavelength, same frequency and fixed phase difference.
Define interference
The pattern observed when two or more waves superpose with each other (could be the pattern of light, sound, microwaves etc).
What is the fundamental frequency equation in symbols?
v/2L or (√ (T/μ))/2L
What factors affect the speed of a wave on a string?
Tension.
Mass per unit length.
What is meant by the intensity of wave?
The power transferred by a wave per unit area.
What is the relationship between intensity and amplitude of a wave?
I α A^2
What is the equation for the intensity of a wave?
I = P/A
What are the properties of all electromagnetic waves?
All of them are transverse.
All of them travel at the same speed in a vacuum.
All of them can be refracted, reflected, diffraction, and undergo interference.
All of them obey the relationship v = fλ.
All of them carry energy.
All of them consist of vibrating magnetic and electric fields at right angles to each, and to the direction of travel.
What can happen when waves meet an interface between two materials?
They reflect, refract, transmit and absorb.
Define interface.
The boundary between two different media.
Define refractive index.
A measure of how much the speed and direction of light changes between two different
media.
How can you calculate refractive index?
c / v
“c” being the speed of light
“v” being the speed of light in the material.
Describe how to find the refractive index of a glass.
Use a “ray box”/”LVU” with a slit screen and lens. Shine the ray through a glass block at an angle. Draw crosses on the rays entering and leaving the block. Join up the crosses to show the three rays. Measure the angle of incidence and angle of refraction when entering/leaving the block. Use the equation for Snell’s law (where the refractive index of air = 1) to find the refractive index of glass. Or, take readings for different angles and plot a graph of sin i / sin r. The gradient will be the refractive index.
How do you reduce uncertainty in finding the refractive index of a solid material
The lens/slit screen makes the beam narrower which reduces the percentage uncertainty in the position of the beam.
Using larger angles rather than smaller ones reduces the percentage uncertainty in the angle measurements.
What is the critical angle?
The greatest angle of incidence in an optically dense medium where the angle of refraction is 90°
What is Snell’s Law?
n1 sinθ1 = n2 sinθ2
What is total internal reflection?
When the incident ray is greater than the critical angle all light is reflected back into the material.
Under what conditions does total internal reflection occur.
Light must be travelling from a more dense medium into a less dense medium (i.e. into a medium of lower refractive index).
How could you predict if total internal reflection will occur at an interface?
sin C = 1 / n
What is the principle focus?
In a converging lens, all rays converge onto the point called the principle focus.
In a diverging lens all (virtual) rays appear to have diverged/come from the principle focus.
What is meant by the focal length of a lens?
The distance between the lens and the principle focus.
What does it mean if the focal length of a lens is negative?
The lens is a diverging lens.
Why is a converging lens’ focal length always positive?
Because the principal focus is always in front of the lens.
What are the properties of a converging lens?
It is also called a “convex lens” and the image is always real and inverted except when the object is in front of the principal focus.
What are the properties of a diverging lens?
It is also called a “concave lens” and the image is always smaller, virtual and upright wherever the object is positioned.
State the lens equation.
1 / f = (1 / u) + (1 / v)
“u” being the distance between the object and the lens.
“v” being the distance between the image and the lens. It is also positive if the image is real and negative if the image is virtual.
“f” is only negative for a diverging lens.
Under what conditions is the lens equation true?
The lens must be thin.
What does the power of a lens tell you?
The higher the power of the lens, the more it will refract light.
What is power measured in?
Dioptres (D)
For diverging lens it is -D
How can you calculate the power of lens?
P = 1 / f
How can the power of a compound lens be calculated? (lots of thin lenses in combination).
P = P1 + P2 + P3 + P4 + …
The lenses must be very close together/touching.
How can you calculate magnification?
m = v / u
hi / ho (image height / object height)
What is a polarised wave?
A wave that only oscillates in one direction/one plane of polarisation which includes the direction of energy transfer.
What is perpendicular to an unpolarised light wave?
An electric and magnetic field.
What waves can be polarised and why can light waves be polarised?
Only transverse.
Light is a mixture of many different oscillations of vibrations and can be polarised, which is evidence that light is a transverse wave.
Describe what happens when two polarising filters are used and one is rotated.
The first filter polarises the light and all components of oscillations perpendicular to the plane of polarisation are absorbed.
Initially, the second filter is aligned so the planes of polarisation of both filters are parallel. This is when the light is most intense.
As the second filter is rotated, the amount of light that passes through it reduces.
If the second filter is 90° to the first, no light will pass through as it is all absorbed and the light is least intense.
As you continue to rotate the filter the amount of light passing through the filter combination will then increase.
When the total rotation of the second filter is 180°, the planes of polarisation of both filters are parallel again.
Why is reflected or refracted light partially polarised?
Because most of the vertical oscillations are absorbed which leaves mostly the horizontal oscillations/planes.
Why do TV and radio aerials have to be carefully aligned?
TV and radio aerials are polarised by the orientation of the rods on the aerial. The rods are either aligned in the same plane either horizontally or vertically depending on the transmitting aerial they are pointing at.
To receive a strong signal, you must line up the rods on the receiving aerial with the rods on the transmitting aerial. If they are not aligned the signal strength/intensity will be lower.
If the TV or radio aerial is moved around after tuning it, the signal will come and go, as the transmitting and receiving aerials go in and out of alignment.
How are microwaves polarised?
The wavelength of microwaves is significantly longer than the wavelength of visible light, so the polarising filters they are used with will not work for light. Instead, microwaves can be polarised using metal grilles.
When the plane of polarisation of the metal grilles is at right angles (90°) to the plane of polarisation of the microwaves, the intensity drops to zero, because the grille is absorbing all of the energy.
What is diffraction?
When a wave is spread out after passing through a narrow gap or around an obstacle.
What waves can diffract?
All of them.
What is Huygen’s construction?
A model for predicting the future movement of waves/a wavefront if we know the current position of the wavefront.
How is Huygen’s construction used to explain diffraction.
Every point on a wave front may be considered to be a point source of secondary spherical wavelets that spread out in the forward direction with the wave speed. The new wavefront is the tangential line that joins all of these secondary wavelengths.
Using Huygen’s construction shows how, if the gap is the same size as the wavelength there is more diffraction. If the gap is bigger than the wavelength there is less diffraction/spreading out.
Why is it easier to diffract sound than light?
The wavelength of visible light is small, so the gap needs to be similarly small to diffract it (i.e. a diffraction grating). The wavelength of sound is longer so the gap can be larger and it is easier to diffract. This is why sound can diffract through a doorway, but light cannot.
What would be seen on a screen if laser light is passed through a diffraction grating?
A series of monochromatic (since laser light is monochromatic) bright, single maxima spread out horizontally where it is brightest at the centre.
What would be seen on a screen if white light is passed through a diffraction grating?
A bright/high intensity central maxima (white light as it is not diffracted) followed by maxima on either side. Each maxima is a spectrum. The coloured light is diffracted by different amounts due to the different wavelengths of each colour.
What is the diffraction grating equation?
dsinθ = nλ
“d” is the distance between the slits on the grating.
“n” is the order of the spectrum spot.
“θ” is the angle between the zero order and the n order.
“λ” is the wavelength of the wave.
Why when a laser beam is shone through a diffraction grating is a pattern shown?
As light passes through two slits it diffracts, producing a coherent source from each slit. The two coherent waves meet together on the screen, where there is interference, either constructive (forming a maximum) or destructive (forming a minimum).
To form a bright spot, the waves superpose and the displacements of the two waves sum to produce a maximum.
The path difference between the two waves will be a whole number of wavelengths, nλ, where n is an integer.
This produces a phase difference of 2π showing that they are in phase.
What would happen to the pattern displayed by a diffraction grating if the split spacing or wavelengths increased or decreased?
If d increases, sin θ becomes smaller, θ becomes smaller so the pattern is less spread out.
If d decreases, sin θ becomes larger, θ becomes larger so the pattern is more spread out.
Sin θ increases therefore θ increases so the pattern will spread out/maximas will be further apart.
Sin θ decreases therefore θ decreases so the pattern will be less spread out/maximas will be closer together.
Which experiments provide evidence that waves behave as waves.
Reflection, refraction, interference, diffraction, I α A^2.
Which experiments provide evidence that waves behave as particles?
Photoelectric effect, line spectra.
Which experiments provide evidence that particles behave as particles?
Conservation of energy, conservation of momentum.
Which experiments provide evidence that particles behave as waves?
Electron diffraction.
The pattern of light and dark bands/rings is consistent with what would be observed in light passing through a diffraction grating.
Describe the electron diffraction experiments.
A filament (the cathode) is heated and emits electrons by thermionic emission.
These electrons are accelerated towards an anode by a large accelerating voltage.
There is a hole in the anode to produce a fine beam of electrons that hit the thin polycrystalline graphite target.
The experiment is in a vacuum, to prevent collisions between the electrons and particles in the air.
Electrons that hit the fluorescent screen cause visible light to be emitted.
A diffraction pattern is observed on the fluorescent screen of a series of light and dark rings.
What is the de Broglie wave-particle duality theory?
Louis de Broglie discovered that electrons could show wave-like properties. The de Broglie equation links a wave property the (de Broglie wavelength) with a moving particle property (momentum).
λ = h / p
The faster the electrons, the greater their momentum and the shorter their de Broglie wavelength.
What happens to the diffraction pattern seen if the accelerating voltage in the electron diffraction tube is increased?
As the accelerating voltage increases, the velocity of the electrons increases, the diameter of the rings decreases, therefore their de Broglie wavelength must decrease.
Why is electron diffraction useful to investigate crystalline structure, but cannot be used for something like a car?
Diffraction only occurs if the (de Broglie) wavelength is the same size as the gap/space it interacts with.
The de Broglie wavelength of electrons is similar to the spacing between the layers of atoms in graphite. So, diffraction is observed.
For particles with more mass (e.g. a tennis ball), the gap needed is too small to be possible, or it would need to be moving too slow to be possible for the gap to be similar to the de Broglie wavelength.
What is an ultrasound transducer?
It can both transmit and receive ultrasound pulses at the same time.
What happens when ultrasound meets a boundary between two media?
The boundary between two media is called an interface. When waves (e.g. ultrasound) meet an interface, some energy is transmitted, some is reflected, and some may be absorbed.
The amount of energy transmitted/reflected/absorbed depends on the difference in densities between the two materials.
What is the pulse-echo technique?
A short pulse of ultrasound is produced by a transducer. It passes through the body (for example). When it meets an interface, some is reflected back to the transducer. The transducer records the time it takes for the pulse to return. This is used to calculate the distance (using v=s/t). Note, the distance is there and back, so the time should be halved.
A computer uses this information to build an image of different tissues in the body.
Used for ultrasound foetal scans, sonar in boats.
What does resolution mean?
The ability to distinguish one object from another.
Describe and explain how to ensure a clear ultrasound image.
Gel is applied to the skin to remove air between the transducer and the skin because air has a substantially lower density than skin, so almost all the ultrasound would be reflected at the skin surface. This increases the proportion of ultrasound waves entering the body.
How does pulse duration and wavelength affect the image produced in the pulse-echo technique?
The ultrasound transducer cannot transmit and receive pulses at the same time – there must be a gap in time between them (called the listening time). Shorter pulses mean it is less likely that the transmitted and reflected pulses will overlap. The listening time is longer. This increases the amount of information that can be gathered, making the image clearer.
For an object to be resolved, the wavelength of the ultrasound must be similar size to the object. Shorter wavelengths diffract less, so produce clearer, higher resolution images.
Define a photon
A discrete packet/particle of electromagnetic radiation.
What are the two equations to calculate the energy of a photon?
E = h f
E = (h x c) / 𝜆
What is the photoelectric effect?
When electrons are released from the surface of metal due to incident electromagnetic radiation. There is a 1 to 1 interaction between a single photon and an electron. The energy of the photon depends on its frequency (E = h f). If the frequency of the photon is above the threshold frequency of the metal, electrons will gain enough energy to be released OR If the photon energy is greater than the work function, electrons will be released.
What is a quantum?
A “packet”.
What is a photoelectron?
An electron released from the surface of a metal due to incident electromagnetic radiation.
Define work function.
The minimum energy required to release an electron from the surface of a metal.
Define threshold frequency.
The minimum frequency required to release an electron from the surface of a metal.
Define threshold wavelength
The maximum wavelength required to release an electron from the surface of a metal,
How can you calculate work function?
𝜙 = ℎ 𝑓𝑜
𝜙= ℎ𝑐 / 𝜆𝑜
2nd equation is found by rearranging 𝑐=𝑓𝜆 for f, and substituting this in.
What is Einstein’s photoelectric effect equation?
ℎ 𝑓 = 𝜙 + 0.5mv^2
(Photon energy = work function + Ek)
What is the electronvolt?
An electronvolt is a measure of energy. It is much smaller than a joule.
How can you convert between electronvolts and joules?
1eV = 1.6 x 10^-19 J
What is the effect of increasing the intensity of radiation below the threshold frequency?
Even though more photons hit the surface per second, per m^2, no photoelectrons are released/it has no effect, because the photons do not have enough energy to overcome the work function.
What is the effect of increasing the intensity of radiation above the threshold frequency?
More photons hit the surface per second, per m^2, therefore more photoelectrons are released per second, per m^2. The maximum kinetic energy of the photoelectrons stays the same as the photons do not have more energy.
Why is it important that air is not allowed to enter the photocell?
Air oxidises the metal plate. This increases the work function of the metal. So, less photoelectrons will be released, and the current will be less
Photons are absorbed by the air. Less photons hit the metal plate. Therefore there are fewer one to one interactions between photons and photoelectrons. Therefore less photoelectrons are released, and the current is less
Photoelectrons collide with the air so less photoelectrons reach the anode therefore the current is less
What does the graph from the photoelectron experiment show?
The threshold frequency is the y intercept.
The gradient is “h” (Planck’s constant).
Why does the photoelectric effect provide evidence for the particle nature of electromagnetic radiation?
Light travels as particles, called photons. The energy of the photons is linked to the frequency of EM radiation. There is a 1 to 1 interaction between a single photon and an electron where the photon is absorbed, and the electron gains the energy. If it gains energy lower than the work function, the metal will heat up. If it gains energy more than the work function, it will leave the surface of the metal. This supports the experimental evidence from the photoelectric effect.
Why did the wave theory of light not support the photoelectric effect?
According to wave theory, energy α intensity α A^2. Wave theory states that brighter, more intense light (with a bigger amplitude) has more energy. If wave theory was correct, the energy would be spread out over the surface, and each electron could gradually accumulate energy until there is enough to emit an electron. If wave theory was correct, the greater the intensity of the light, the more energy it has so the kinetic energy of the electrons should increase. This is not supported by the photoelectric effect, where intensity does not affect the kinetic energy of the photoelectrons. Wave theory also does not explain the threshold frequency where electrons should be emitted at all frequencies.
What happens to the gold leaf when the zinc plate gets negatively charged?
The gold leaf falls because the photos of UV have enough energy to overcome the work function. Photoelectrons are lost from the zinc and the overall excess negative charge on the electroscope decreases, therefor the repulsive force between the two parts decreases. Hence the leaf falls.
What happens to the gold leaf when the zinc plate gets negatively charged?
The gold leaf stays in its original position. No photoelectrons are lost from the zinc. As the plate is positively charged, there is a strong attractive force keeping the negatively charged electrons on the plate. The photons do not have enough energy to overcome this.
Why is it the maximum kinetic energy of photoelectrons in Einstein’s equation?
Work function is the minimum energy needed to release an electron from the surface of a metal. This would correspond to the maximum kinetic energy of photoelectron near the surface. Electrons deeper down in the metal will need more energy than the work function, so will be emitted with less kinetic energy.
What does ground state mean?
The lowest energy level of an atom.
How can an electron move up and down energy levels?
Electrons can move up and down energy levels (transitions) by either losing exactly the right amount of energy, or gaining exactly the right amount of energy. This energy is in the form of a photon.
ΔE = E2 – E1 = hf = hc/λ
Describe and explain how a continuous spectrum could be produced.
White light is passed through a prism or a diffraction grating. The different colours of light refract (prism) or diffract (grating) by different amounts.
The spectrum is continuous – there are no gaps.
Describe and explain how line emission spectra are formed.
A glass tube/source contains atoms of only one type of gas. These atoms are ….
excited due to high temperatures
this causes their electrons to move to higher energy levels
the electrons at these higher energy levels are unstable
they de-excite/relax to a lower energy level. To do this they emit a photon of energy equal to the difference in energy between the two energy levels
A line emission spectra consists of a series of bright lines against a dark background, some of which may not be visible light
The line emission spectrum only has certain photon energies present, therefore only certain wavelengths are being emitted
Each line corresponds to a photon emitted when the electron de-excites between one energy level and another
There are discrete energy levels in an atom, so only certain transitions are possible, so only certain photon energies are possible corresponding to specific wavelengths
Describe and explain the line absorption spectra.
White light is passed through a cool gas
At low temperatures, most of the electrons in the gas are in the ground state
Photons of the correct energy and wavelength (that correspond to the difference in electron energy levels for the atom) are absorbed by the electrons in the cool gas
The electrons are excited to a higher energy
Those energies/wavelengths are missing from the continuous spectra as they have been absorbed
How do line emission and line absorption spectra compare?
The bright lines present in the emission spectra match the lines absent in the absorption spectra.
