Turning points

What was the aim of Michelson-Morley experiment?

To prove that there was ether by showing that the speed of light changed with different rotations in respect to the ether.

What was the expectations of the Michelson-Morley experiment?

It was expected that when rotating the blocks and the mirrors by 90 degrees, the interference pattern seen should change as

the speed of light should be changing in respect to the ether.

What was the significance of the failure of the Michelson Morley experiment?

The interference pattern didn’t change with the rotation therefore concluding that the speed of light is constant and

there is no ether.

What is the two postulates of Einstein’s theory of special relativity?

- Physical laws have the same form in all inertial frames.

- The speed of light in free space is invariant (always c for all observers).

What is time dilation?

Time runs slower for an observer when observing a moving object that it does for the object itself. This effect increases

greatly as you approach the speed of light.

Why is time dilation a consequence of special relativity?

Since the speed of light is the same in all frames, the relative speed of an object moving at the speed of light would vary to that of another observer, observing both objects and their relative speeds.

What is proper time?

The time observed by the object, or an observer that is stationary with respect to the moving object.

Known as t₀.

How does muon decay provide evidence for time dilation?

The intensity of muons with distance decreased less than expected. This is because the half life to human observers is much larger than the actual half life of the muon due to time dilation.

How does special relativity effect lengths?

Length contraction is another conclusion to be drawn, an observer parallel to an moving object will observe a shorter

length than an observer at rest relative to the rod.

What is wave particle duality?

The idea that particles can also behave like a wave.

Outline Newton’s corpuscular theory?

That light is made up of small particles. He proposed that white light is made up of different colours which he observed when he split light put in a prism.

Outline briefly Huygens wave theory on light?

● States that all of the points on a wave front are sources for secondary wavelets.

● Light was a longitudinal wave but as longitudinal waves must travel through a medium and space was deemed to be ‘empty’ Huygen also suggested that space was filled with aether, a substance we cannot see and that has no mass.

Explain reflection in terms of Newton’s corpuscular theory.

Newton suggests that the particles are like balls, and when they bounce of the surface the ‘reflect’ due to a conservation of momentum between the surface and the particles. As the speed only changes in the direction perpendicular to the surface then it must be reflected at the same angle.

Describe the difference between Newton’s theory and Huygens theory.

on refraction.

Newton suggested that light diffracts due to the material exerting an attractive force on the light particles causing them to increase vertical speed as it entered the material. The change is speed is what leads to the change in angle when entering/leaving the medium.

Whereas Huygen’s theory suggests that the wavefronts hit the boundary at an angle. This causes the light to take infinite amount of time to slow down at the boundary.

Which theory of light was preferred? Newton’s or Huygen’s? Why?

Newton’s was preferred. This is because Newton had a better reputation at the time than Huygen and was seen to be more

important.

How did Young’s double slit theory support Huygen’s wave theory.

● The double slit theory displayed a pattern of fringes due to interference between different wavefronts.

● Young’s double slit theory occurs due to diffraction and only waves can diffract.

● Therefore suggesting that light is a wave like Huygen suggested.

How are fringes formed in Young’s double slit?

● Monochromatic light passes through two slits.

● The two slits act as coherent light sources.

● This causes bright fringes where the light is in phase and constructive interference occurs.

● Dark fringes where destructive interference occurs as the light is out of phase.

Why can Newton’s theory NOT explain Young’s interference pattern?

Because if light acted like particles like Newton suggested then there would be two light fringes where the slits are, not a repeating pattern of light and dark fringes.

Why was Newton’s theory eventually rejected?

Newton’s theory of light was rejected many years after Young’s discovery of interference, in which the speed of light in water was measured and found to be less than the speed in air.

What were Maxwell's predictions about electromagnetic waves?

He predicted that an electromagnetic wave has an electric and magnetic that oscillate perpendicular to each other. He also derived an equation for the speed of an electromagnetic wave. He showed that light and electromagnetic waves travelled at the same speed in a vacuum therefore showing that light is a EM wave as well as ultraviolet, infrared and others.

What was Maxwell’s equation to calculate the speed of light?

c = 1/√(ε_0 • μ_0 )

What does ε_0 represent? And what does it relate to?

It represents the permittivity of free space and it related to the strength of the electric field as a result of a charged

object in free space.

What is μ_0 ? What does μ_0 relate to in the equation?

It stands for the permeability of free space, which is related to the magnetic flux density because of a current carrying wire in free space.

How are stationary waves produced when radio waves are reflected?

The reflection produces two coherent progressive waves travelling in opposite directions. The two waves interfere and cause constructive interference at antinodes and destructive interference at nodes.

How did Hertz discover radio waves?

Hertz created electromagnetic waves by producing them when a high voltage spark jumped from one metal plate to another. The spark produced radio waves which he detected using a dipole detector. As the radio waves produced a voltage in the detector which causes a spark in the detector.

What properties did Hertz find out about radio waves in further experiments?

● They can be reflected by a metal sheet.

● They can produce stationary waves.

● Radio waves cannot be stopped by insulators.

● Radio waves are polarised.

How did Hertz use his experiment to work out the speed of radio waves?

When he produced the stationary waves, Hertz measured the distance between adjacent nodes on the wave (which is 1/2λ). He then used the equation c = fλ. The value he got was similar to Maxwell’s predicted value from his equation.

True or false? ‘All objects that have a temperature above zero will emit infrared radiation’

True.

What is a black-body?

An object that emits and absorbs all parts of the EM spectrum.

What is the ultraviolet catastrophe?

It was a disagreement between the practical and theoretical predictions of the intensity of energy at different wavelengths.

The theoretical prediction suggested there was an infinite amount of energy at the very low wavelengths.

How did Planck solve the ultraviolet catastrophe?

He suggested that EM radiation is released in photons (which are related to a frequency) and not continuous therefore it is not infinite.

What is the equation to work out the energy of a quantum of EM energy?

E = hf

Where h is Planck’s constant.

When was photoelectric emission first discovered?

When Hertz was carrying out experiments with radio waves. He observed that the strengths of the sparks changed with different EM waves, however if the same EM wave was incident on the detector for a extended period of time then it wouldn’t cause the strength the vary.

If the frequency of light incident on a metal is below the threshold frequency,

can photoelectrons still be admitted?

No it can not, because the low frequency will mean that each photon of light would not provide the electron with enough energy for the electron to leave the atom.

What is the maximum kinetic energy of photoelectrons dependant on?

The frequency of the light used.

How do you calculate the energy of a photon?

E = hf

What is the photoelectric equation?

hf = ⲫ + 1/2 • m • v^2

Where ⲫ is work function

How does increasing the intensity of light (with a frequency above the threshold) effect the photoelectric emission?

A higher intensity means more photons therefore more photon- electron interactions. Which means more electrons are emitted.

Why does the photoelectric effect not support wave theory?

Because the photoelectric effect causes an instantaneous release of electrons if the frequency is about a certain frequency which is explained by photons. However wave theory suggests the emission won’t always happen instantaneously and predicts that any frequency of light will eventually cause the photoelectric effect to occur, as while the wave incident on the metal the electrons gain energy until they have enough to escape.

What is the stopping potential?

The potential required to stop electrons.

There at the stopping potential(V_s):

eV_s = KE_max

How do you measure the stopping potential?

You use a photocell which has an anode and cathode. When certain frequencies of light are incident on the anode, releases photoelectrons. These electrons then travel from the anode to the cathode producing a current. A potential difference is put across the plates, making the anode more positive and making it harder for electrons to leave the anode. The stopping potential is the potential needed to ensure that no electrons reach the cathode, meaning no current running.

What is wave particle duality?

The idea that all particles can behave like waves as well as particles.

What pattern was observed in electron diffraction and why is the pattern seen?

A pattern of concentric rings/diffraction rings are seen. This is due to the constructive and destructive interference that occurs from when the electron beam is diffracted.

Why was the experiments on electron diffraction significant?

Because, only waves were known to diffract. Therefore showed that electrons (particles) could also behave like waves as de Broglie previously suggested.

How is the beam of electrons used in an electron diffraction experiment created?

It is created by producing electrons through thermionic emission and accelerating the electrons from rest using a potential difference. This means that once fully accelerated, all electrons will have the same kinetic energy which is equal to the work done by the potential difference: 1/2 • m • v^2 = e • V

What is de Broglie’s wavelength in terms of anode potential (V_A )

Λ = h/√(2 • m • e •V)

How does increasing the electron speed change the diffraction pattern?

If you increase the electron speed, then you will decrease the wavelength of your wave, which will mean the diffraction rings would appear closer together.

What anode voltage is required for electrons to have a wavelength of the

same order as an atom?

150 V

What is resolving power?

An object's ability to acknowledge two seperate objects close together are seperate objects, not just one.

How are images formed in the TEM (Transmission electron microscope)?

● An electron beam produced by thermionic emission is accelerated using a anode.

● There is than a magnetic lenses which focus the electrons. Meaning any electrons that were falling at an angle near the edge are now are deflected to fall to the centre. The electrons already passing through the centre are unaffected.

● The electrons are then focused on a sample by a condenser lens and pass through our sample.

● There is then an objective lens which forms the image of the sample.

● Then the projector lens creates the final image on a fluorescent screen.

How does decreasing the anode potential affect the detail of the image?

A lower anode potential would mean the electrons gain less kinetic energy, which would therefore mean their wavelength will increase and the resolving power would therefore decrease making it less detailed.

How does the thickness of the sample limit the detail of the image?

Because when the electrons pass through the thick sample the electrons slow down, which causes the wavelength to decrease and reducing the detail.

How can lens aberrations limit the detail of an image from a TEM?

The electrons are travelling at different speeds due to being scattered. Therefore the lens can’t focus and electrons from each point of a sample to a point on the screen.

Why is the pressure inside the TEM really low?

In order to reduce the amount of collisions that could occur between the electrons and air particles. If the pressure was high then there would be more frequent collisions between the electrons and air particles, which would cause the transfer of energy making the electrons lose energy. This would therefore decrease the detail of the image.

How does an STM (scanning tunnelling microscope) work?

A STM creates images using quantum tunneling, a fine probe is placed close to the surface of a material to create a tunneling current between the surface and the probe. A small pd goes between the probe and the surface so electrons cross the gap from negative to positive.

What is an ether (aether)?

A substance that was thought to have filled space. Scientists hypothesised that it was the matter that allowed light - as a longitudinal wave - to move through space.

What is meant by thermionic emission?

A filament is heated up using an electric current, this causes the delocalised electrons to gain energy. Eventually they gain enough energy for the electron to leave the metal surface as a beam of electrons.

How are cathode rays made in a discharge tube?

● Electrons are released by thermionic emission.

● The electrons are repelled by the cathode and accelerated towards an anode.

Why is light emitted from a discharge tube?

The strong potential difference between the anode and cathode cause atoms in the discharge tube to ionise. When this occurs the ionises atoms (which are now positive ions) are attracted to the cathode. They accelerate towards and then collide into the cathode. When this happens electrons leave the cathode and go on to excite other atoms. When these atoms de excite they release photons of light.

What is the speed, v, of each electron leaving the anode in a cathode ray?

The work done on each electron by the potential difference V between the anode and the cathode is eV (e - electron charge).

The kinetic energy of each electron, with speed v, passing through the hole is ½ mv^2

The work done on each electron increases KE at the cathode, then the speed, v, of each electron leaving the anode is given by ½ mv^2 = eV. So v = sqrt(2eV/m)

How do you work out the specific charge of an electron?

Specific charge of an electron = The charge of an electron / The mass of an electron

State 3 methods used to work out the specific charge of an electron.

● Using a magnetic field

● Using m = mv / Be

● Using e / m = v / Br

Why must electron tubes be evacuated when working out specific charge?

So the electrons do not collide with air particles and lose energy in the collisions.

If gas is pumped into a electron tube, why must the pressure be low?

A low pressure means less molecules which is beneficial because too many gas molecules could disrupt the path of the electrons. This is because the more air particles, the more interaction between them and the electrons. Which could mean the electrons won't be able to travel the whole length of the tube.

Who was Thomson?

A physicist who conducted experiments to investigate cathode rays, and the particles that are produced. This included him carrying out experiments determining the specific charge of an electron.

Why was Thomson’s experiments important?

● Showed that electrons were negatively charged.

● Showed that the specific charge of a particle is a characteristic of that type of particle as all electron has the same specific charge.

● The specific charge was very high which showed that an electron had to have little mass.

What was the aim of Millikan’s experiments?

To determine the charge of the electrons.

In Millikan’s experiment, what forces are acting on the droplet when it is stationary?

Gravity and an electric force which is equal and opposite to the gravitational force.

In Millikan’s experiments, explain the journey of a falling droplet when there is no electric field?

The droplet will begin falling and accelerating as it does the drag acting on it will increase. Eventually the weight will equal drag force and the droplet will fall at terminal velocity.

What is Stokes’ law?

F = 6πηrv

Which is used to work out the force on a droplet due to drag.

How did Millikan use the value of the radius to determine the charge of an electron?

He used the radius to determine the mass of the droplets. That way he could calculate the weight of a droplet. He then know the pd required for an object to remain stationary. Therefore the charge could be worked out using QV/d = mg.

How did Millikan cause the droplet to move down from stationary in the

presence of an electric field?

Milikan decreased the pd which would have reduced the electric field. Therefore unbalancing the forces and making the resultant force on the object act downwards.

Why was Millikan's experiments significant?

It helped him to conclude that charge of an electron, by assuming that the charge on each droplet would be a multiple of the charge of an electron. He found the common factor between the different charges found that there were no charges less than approx 1.6x10^-19 Millikan showed that the charge on all material is quantised.

Thermionic emission

Thermionic emission is where a metal is heated until the free electrons on its surface gain enough energy and are emitted.

Fine beam tube

This piece of apparatus contains a low pressure gas and has a uniform magnetic field passing through.

Atomiser

An atomiser is used to spray tiny droplets of oil, which are negatively charged due to friction.

Reflection

The corpuscles collide with the surface and a repulsive force pushes them back, causing their component of velocity perpendicular to the surface to change direction, while their component of velocity parallel to the surface stays the same.

Refraction

As the corpuscles approach a denser medium, short-range forces of attraction cause their component of velocity perpendicular to the surface to increase, while the parallel component of velocity stays the same, therefore the light will bend towards the normal.

Electromagnetic waves

Electromagnetic waves are formed of an alternating magnetic and electric fields travelling in phase and at right angles to each other.

Black body

A black body absorbs and emits all possible wavelengths of radiation.

Electron diffraction

Electron diffraction provided experimental evidence for the de Broglie hypothesis as it showed that electrons, which are particles, can also undergo diffraction, which can only be experienced by waves.

Two types of electron microscope

1. Transmission electron microscope (TEM).

2. Scanning tunnelling microscope (STM).

Condenser lens

This is the first lens the electrons beams pass through and this lens deflects the electrons so that they form a wide parallel beam, which is directed at the sample.

Objective lens

This lens will form an image of the sample, which is directly above it.

Projector lens

This will magnify the image made by the objective lens and project it onto the fluorescent screen.

Sample thickness

As electrons pass through the sample they will slow down, causing their wavelength to increase and so the resolving power is decreased.

Electrons travelling at a range of speeds

As the electron gun emits electrons through thermionic emission some electrons may lose kinetic energy while leaving the metal (due to collisions). This leads to electrons travelling at different speeds, having different wavelengths and therefore being diffracted by different amounts which causes blurring of the image (known as aberration).

Muon decay

Muon decay provides experimental evidence for time dilation because muons enter the atmosphere at very high speeds and so experience significant time dilation, which affects how quickly they decay.

Length contraction

Length contraction is a consequence of special relativity, meaning it only occurs in inertial frames and causes the length of objects moving at high speeds to appear shorter to an external observer

High voltage

A high voltage is applied across the gas - this produces a strong electric field which pulls electrons from the gas atoms, causing them to become ionised.

Negative glow

The negative glow is produced when positive ions rejoin with electrons near the cathode and release photons in the process.

Positive glow

The positive glow is produced when cathode rays move towards the anode and cause excitation of gas atoms - when these atoms de-excite, photons are emitted and produce the ‘positive glow’.

Specific charge

Specific charge is the ratio of charge to mass.

Specific Charge = Q/m

Reflection

Reflection was thought to be caused by the corpuscles reaching the surface and being acted on by a repulsive force that changed their direction.

Refraction

Refraction was thought to be caused by a perpendicular force acting on the corpuscles as they enter a new medium, causing a perpendicular acceleration - the parallel component of the corpuscles’ motion remains unchanged

Two postulates for Special Relativity.

1. The laws of physics have the same form in all inertial frames.

2. The speed of light is invariant.

Black body

A black body is a body that emits all the possible wavelengths of radiation for the temperature that it’s at.

Quantum tunneling

Quantum tunneling is the idea that a wave’s amplitude isn’t always reduced to zero if passing through a thin barrier. It is applied to matter waves, and explains why electrons can ‘tunnel’ across the small gap.

Anode

A positively charged electrode.

Bertozzi’s Experiment

An experiment that investigated the velocity of electrons emitted with different kinetic energies. It showed that the speed of light was never exceeded and that as the kinetic energy continually increases, the velocity of the electrons continually approaches the speed of light.