Phy5A

Front 1

Newton's explanation for reflection

Back 1

- Incoming particle speeds up as it gets attracted by the surface

- Bounces away then gets slowed down by the same force so goes off at the same speed

- Corpuscles bounce off the mirror without loss of speed during reflection

- Magnitude of velocity of normal and parallel components are unchanged on reflection, which means angle of reflection = angle of incident

Front 2

Newton

Back 2

- Light consisted of particles (corpuscles) which travel in a straight line

- Corpuscles travel faster in more dense material

- Corpuscles require a media to propagate

- Reflection is easy to explain

- Refraction more difficult (he said the light was attracted to more dense materials)

- Interference and diffraction can't be explained. (Young's slits, for example, result in two bright fringes only)

- Able to explain polarisation

Front 3

Huygens

Back 3

- Thought that light was a wave

- Light waves travel slower in more dense material

- Light waves are longitudinal (so can't explain polarization)

- Refraction can be explained (but speed of light can't be measured)

- Interference can be explained

Front 4

Newton's explanation for refraction

Back 4

- Corpuscles travel faster in substance than air because they're attracted to it

- The incoming particle is accelerated towards the surface and so changes direction, decelerates as it moves away

- The component of velocity perpendicular to the boundary of each corpuscle is increased as the corpuscle crosses the boundary into the substance

- The component of velocity parallel to the boundary is unchanged

Front 5

Huygen's explanation for reflection

Back 5

Wavelets travel back from the reflected wave, interfering with the other wavelet around them, resulting in a flat wavefront

Front 6

Huygen's explanation for refraction

Back 6

- Assumed that light travels slower in a transparent substance than air

Front 7

The wave theory of light was rejected by most scientists in favour of Newton's corpuscular theory because:

Back 7

- Wasn't possible to measure speed of light in air or in a transparent substance at the time

- No experimental evidence to confirm whether light travels faster or slower in a transparent substance than air

- Newton had better reputation so the community was biased

- Wave theory of light was considered in terms of longitudinal waves; couldn't explain polarisation of light

Front 8

The wave theory of light was not accepted even after Young demonstrated interference of light through double slits

Back 8

Newton's theory of light was only rejected in favour of wave theory long after Young's discorvery of interference when the speed of light in water was measured and found to be less than the speed in air

Front 9

The nature of electromagnetic waves consist of oscillating electric and magnetic fields right-angle to each other

Back 9

- Maxwell showed that speed of electromagnetic waves in free space, c, is given by
which is the same as the speed of light in free space

- He showed that light is an electromagnetic wave and infra-red and UV radiations beyond the visible spectrum are also electromagnetic waves

Front 10

Hertz's showed that radio waves are produced when high voltage sparks jump across an air gap which could be detected using

Back 10

- A wire loop with a small gap in it

or

- a 'dipole' detector consisting of two metal rods aligned with each other at the centre of curvature of concave reflector

Front 11

Hertz

Back 11

- Found that radio waves are reflected by a metal sheet

- Discovered that a concave metal sheet placed behind the transmitter made the detector spark stronger

- Produced stationary radio waves by using a flat metal sheet to reflect from their frequency and the distance between adjacent nodes

- Discovered that insulators do not stop radio waves

- Showed that the radio waves he produced are polarised

- Noticed that there were points at which the sparking was non-existent, or very weak, when he moved his detector between his transmitter and a flat metal sheet

Front 12

Photoelectricity

Back 12

- No photoelectrons emissions if frequency < threshold frequency

- Occurs at instant a light of suitably high frequency is incident on the metal surface

- Photoelectrons have range of KE up to a max value that depends on the type of metal & frequency of the incident light

- No. photonelectrons emitted from the metal surface per second is prop. to the intensity of the incident radiation

- One to one interaction between photons and electrons

- Photon loses all its energy to electron

Front 13

In order for a conduction electron to escape, it needs to:

Back 13

- Absorb a single photon and therefore gain energy hf

- Use energy equal to the work function Φ of the metal to escape

Front 14

According to wave theory:

Back 14

- Light of any frequency should cause photoelectric emission

- The lower frequency of light, longer time taken by e in metal to gain sufficient KE to escape

but

- Effect is only observed when light frequency reaches a value of f0 regardless of its intensity

- Effect is instantaneous regardless of its intensity

- Max KE is independent of the intensity of the incident radiation

Front 15

Wave theory cannot explain:

Back 15

- The existence of threshold frequency

- The instant emission of photoelectrons or their KE

Front 16

The max KE of the emitted photoelectrons can be measured by:

Back 16

- Making the 'cathode' increasingly positive wrt. metal anode used to collect the photoelectrons

- More work done by photoelectrons if it is positively charged
- Max KE of photoelectrons reduced by amount of eV

- No. of photoelectrons emitted per second decreases as the cathode potential is made increasingly positive

- At certain potential(stopping potential), photoelectric emission is stopped because the max KE has been reduced to zero

- 0 = hf - Φ - eV

Front 17

Stopping potential can be measured using a potential divider and a photocell

Back 17

V = (hf/e) - (Φ/e)

For different metal

- Gradient is always the same

- Greater work function, further intercepts are from the origin

Front 18

Explain why measuring the stopping potential in the photoelectric effect is useful

Back 18

- To measure the max KE of emitted photoelectrons

- Increase cathode (positive metal plate) pd relative to metal anode to collect photoelectron

- Photoelectrons must do extra work to move away from the cathode

- Max KE is reduced by an amount equal to eV

Front 19

Transmission Electron Microscope (TEM)

Back 19

- Beam of electrons directed at a thin sample in evacuated tube

- Some electrons scattered by sample structure as they pass through

- Electromagnetic coil acting as "magnetic lens" focus scattered electrons onto a fluorescent screen to form a magnified image at the end of tube

Front 20

Electron gun (TEM)

Back 20

- Produces electrons(thermionic emission) from heated filament

- Accelerates emitted electrons through a hole in metal anode at constant pd(50 to 100kV) relative to filament

- Electrons speed depends on anode potential (relative to filament)

Front 21

Magnetic condenser lens (TEM)

Back 21

Produces magnetic field that forces electrons into a parallel beam directed at very thin sample

Front 22

Objective lens/Eye piece lens (TEM)

Back 22

Deflects scatted electrons so they form an enlarged inverted "first" image of the sample

Front 23

Magnifier lens (TEM)

Back 23

Focuses electrons from central are of first image to form a magnified final image on fluorescent screen

Front 24

Limitations of TEM

Back 24

- Can't look at living cell tissue due to radiation

- Vacuum, doesn't interfere with air particles (prevent collisions with electrons and gas molecules)

- Electrons lose momentum going through sample, increases wavelength, reducing retail

- There's range of electron speeds, hence wavelengths, can distort image

- Focal length of magnetic lens depends on current, in practice there are small variations which leads to slight changes in focal

Front 25

How can resolving power in an electron microscope be increased?

Back 25

- Increase of voltage causes increase of speed (electrons)

- Hence a reduced de Broglie wavelength

- Less diffraction for reduced wavelength

- Higher resolving power(better resolution) if less diffraction

- Increase anode pd also enlarges image on screen

Front 26

Factor 1 of limitations on the amount of detail:

Sample thickness

Back 26

Electrons passing through sample suffer a slight loss of speed, increases de Broglie wavelength slightly, reduces resolving power

Front 27

Factor 2 of limitations on the amount of detail:

Lens aberrations

Back 27

- Magnetic field in the outer&inner parts of the lens may focus electrons from a given point to different positions instead to the same positions, causing image to be blurred.

- Electrons scattered have slightly different speeds due to process of thermionic emission and passing through different thickness of sample, therefore focused differently on screen

Front 28

Scanning Tunnelling Electron Microscope (STM)

Back 28

- Electrons have wave-like nature

- De broglie wavelength sufficiently long to stretch and have small probability of tunnelling across a gap

- Smaller the gap, higher the probability, therefore higher tunnelling current

- Tunnelling current is inversely proportional to the distance of the probe from the surface of the sample (gap)

Front 29

Constant height mode (STM)

Back 29

- Height kept constant regardless if there's a pit/gap or not, current varies as gap width changes

- Measure difference in tunnelling current

- Decrease(increase) of current means gaps widen(narrows)

- No current measured if constant height mode is used for big gap

Front 30

Constant current mode (STM)

Back 30

- Can measure bigger gap

- Tip height altered

- To keep current constant

- As tip move across, the tip height rises(falls) if the surface rises(falls)

- Change in current is used to move the probe vertically upwards or downwards respectively

Front 31

Electrons have insufficient KE to overcome the potential barrier caused by the work function of the metal tip

Back 31

- In effect, the amplitude of the electron wave decreases exponentially in the gap

- The bigger the amplitude of matter wave, the greater probability to find electron (to cross the potential barrier)

The gap is sufficiently small so:

- Amplitude is finite on the other side of gap

- Small changes in gap width produce measurable changes in the number of electrons per second crossing the gap

Front 32

What is the purpose of a compensator (M&M Experiment)

Back 32

To compensate the path difference of the light beam when they are reflected from their corresponding mirror and refracted across the the half-silvered mirror

Front 33

Special relativity's two postulates:

Back 33

- Physical laws have the same form in all inertial frame of reference

- The speed of light in free space is invariant (ie independent of the motion of the light source and the motion of any observer)

Front 34

Consequences of the two postulates:

Back 34

- No experiment can be carried out to tell whether an object is moving or not

- Proved that ether do not exist, since speed of light is the same in all directions

- Nothing can travel faster than the speed of light, no matter how fast you travel the speed of light relative to you remains constant

Front 35

Time dilation

Back 35

A moving clock runs more slowly than a stationary clock

Front 36

Length contraction

Back 36

A rod moving in the same direction as its length appears shorter than when it is stationary

Front 37

Relativistic mass

Back 37

The mass of an object increases with speed

Front 38

Explain how Hertz was able to determine the speed of radio waves and other properties of radio waves

Back 38

- Spark gap generated radio waves(high pd), receiver coil with a gap detects radio waves

- Metal sheet used to reflect radio waves back to detector, forms series of maxima/minima, which are nodes&antinodes, then explanation of standing waves

- Measured wavelength(distance between 2 adjacent nodes=half λ)

- Can control radio waves frequency, c=fλ to determine c

- Showed light can be polarised by rotating receiver 360 degree

- Intensity changes as orientation of detector to transmitter changes

Front 39

Give one reason why electromagnetic radiation is emitted during electron capture

Back 39

- Nucleus might be excited

- Energy given up as EM radiation as they deexcite

Front 40

Explain why emf is induced in the loop(Hertz experiment)

Back 40

- Magnetic wave causes alternating magnetic field/flux through loop

- Induced emf in loop due to changing magnetic flux in loop

Front 41

The alternating emf decreases to zero when the loop is rotated about the axis through 90 until it is horizontal. Explain why the emf is zero when the loop is horizontal

Back 41

- Radio wave polarised

- No magnetic flux passes the loop

Front 42

The speed of an object cannot be greater than or equal to the speed of light yet its KE can be increased without limit. Explain the apparent contradiction that the speed of an object is limited whereas its KE is not limited

Back 42

- As speed approaches c, mass approaches infinite

- Gain of KE causes large gain of mass when v approaches c

- KE=0.5mv^2 valid at speeds <

Front 43

State and explain how the image of an object observed TEM would change in anode voltage was increased

Back 43

- Brighter image because more electrons reach the screen per sec

- More detailed image because de Broglie wavelength would be reduced

- Speed of electrons in increased

Front 44

Use Huygen's wave theory of light to explain the formation of fringes by the double slits.

Back 44

- Waves/wavelets emitted by each slit

- Slit diffracts light

- Two slits ensure light waves are coherent

- Bright fringes formed due to constructive interference(or counter-argument)

- Path difference to a bright fringe = whole λ

- Path difference to a dark fringe = (whole number+1/2) λ

Front 45

Explain what Newton's theory of light would predict for the same experimental arrangement as above

Back 45

- Light made of corpuscles

- Would not be diffracted

- Only passes straight through

- Only two bright fringes seen

Front 46

Reasons why Huygen's wave theory of light did not replace Newton's theory of light when the fringe pattern due to double slits was first observed

Back 46

- Newton's scientific pre-eminence

- No evidence light travelled slower as predicted by Huygen

- Huygen considered light longitudinal, cannot explain polarisation

Front 47

Thermionic emission

Back 47

- Filament emits electrons when heated

- Conduction electrons in metal gain enough KE to escape filament

Front 48

Why is it essential that the container is evacuated during thermionic emission

Back 48

- Electrons stopped by gas atoms

- Collide with gas atoms, lose KE/speed

Front 49

Why anode must be +ve

Back 49

- Electrons carry negative charge, anode be +ve to attract

Front 50

Use Newton's theory of light to explain the refraction of the light ray at the air/glass boundary

Back 50

- Corpuscles

- Attracted towards glass surface

- Velocity normal to surface increased

- Velocity parallel to surface unchanged

Front 51

Explain why electrons can cross the gap between the probe tip and the surface, provided the gap is sufficiently narrow

Back 51

- Electron can behave as waves/tunnel across gap

- Waves can cross narrow gaps/non-zero probability of crossing gap

- Electron wave would be weaken too much by large gap

- Narrower the gap, bigger the probability, the greater the number of electrons per seconds that cross the gap

- Electrons transfer from - to +

- Pd is necessary so electrons cross in one direction only

Front 52

Explain, with the aid of a suitable example, what is meant by an inertial frame of reference

Back 52

- Newton's laws obeyed in an inertial frame

- Inertial frames move at constant velocity to each other

- Object moving at constant velocity is not accelerating, so it has an inertial frame of reference

Front 53

Formation of dark/bright fringes on the screen due to monochromatic light passing through slits

Back 53

- Light passing through each slit diffracted

- Diffracted light from one slit overlaps with light from the other slit

- Bright fringes formed due to constructive interference of light waves

- Dark fringes formed due to destructive interference of light waves/out of phase by 180

- Path difference = λ (light fringe)

- Path difference = (λ+1/2) (dark fringe)

Front 54

Discuss why this double slits experiment's fringe pattern cannot be explained using Newton's corpuscular theory of light

Back 54

- Corpuscles travel in straight line passing through slit form a bright fringe

- Two slits produce only two bright fringes according to corpuscular theory

- but > two fringes are observed

- dark fringes(cancellation) cannot happen with corpuscles

Front 55

Explain why the droplet of oil stopped moving and remained stationary when the potential difference between the plates was adjusted to a certain value, V(c)

Back 55

- Electric force is vertically upward and proportional to plate pd

- At V=V(c) force due to field is equal and opposite to droplet's weight

- No resultant force at V(c), droplet remains stationary

Front 56

When blue light is incident on a certain metal surface, electrons are emitted from the surface. No electrons are emitted when red light, instead of blue light, is incident on the same surface at the same potential.

Use Einstein’s theory of light to explain these observations.

Back 56

- Photon energy = hf

- Blue photon has more energy than red photon

- e absorbs a photon

- e needs a certain amount of energy to escape from metal

- Blue photon gives e enough energy to escape whereas a red photon does not

Front 57

Outline the significance of Einstein’s explanation.

Back 57

- Classical wave theory predicted that all wavelength/frequencies of light should cause electrons to be emitted

- Classical wave theory was rejected in favour of the photon theory

Front 58

(STM) Explain, in terms of wave particle duality, why an electron can cross this small gap

Back 58

- Wave-like nature allows e to transfer

- A wave can penetrate thin barrier/gaps

- Has a non-zero probability of transferring an electron(tunnelling)

- Negligible if gap is too wide

Front 59

Why is it necessary for a potential difference to exist across the gap

Back 59

- With a pd, electrons transfer from - to +

- Pd is needed so a current can flow in single direction

Front 60

Explain why thermionic emission is negligible when the filament current is too low

Back 60

- Filament temperature depends on current

- KE depends on temperature

- Work must be done by e to leave metal

- e have not enough KE to leave

- If current is too low

Front 61

State and explain the effect on the beam of electrons of increasing the filament current

Back 61

- No. of electrons per second in beam increases

- The filament gets hotter, increases temperature, will emit more electrons per second

Front 62

State and explain the effect on the beam of electrons of increasing the anode potential

Back 62

- Speed/KE of electrons will increases

- Electrons from filament are attracted towards anode with a greater acceleration/force

- Gain more KE in crossing a greater pd

Front 63

A beam of electrons is directed at right angles into a uniform magnetic field. Explain why the electrons move in a circular path at a constant speed in the magnetic field.

Back 63

- Magnetic force on each electron perpendicular to velocity

- No work is done on each electron by magnetic force so KE/speed is constant

- Magnitude of magnetic force is constant because speed is constant

- Magnetic force is always perpendicular to velocity so is centripetal

Front 64

Discuss the historical relevance of the value of the specific charge of the electron compared with the specific charge of the H+ ion

Back 64

- Specific charge for electron is ≈ 2000 x specific charge of H+

- Which was the largest known specific charge before the specific charge of electron was measured

Front 65

When the experiment was repeated using a different metal, Y, illuminated by light of thesame wavelength, there was no photoelectric emission when the metal surface wasuncharged.Explain this observation.

Back 65

- Photons have the same energy

- When a conduction electron in the metal absorbs a photon, it gains all the energy of the photon

- Work function is the minimum energy needed by an electron to escape

- Work function of Y is greater than energy gained by an electron (so electron cannot escape)

Front 66

From above

How did this observation contribute to the failure of the wave theory of light

Back 66

- Wave theory predicts that incident light(of any frequency) would cause photoelectric emission (from any metal)

- Wave theory cannot explain why light below a certain frequency(threshold frequency) could not cause photoelectric emission

- This threshold frequency is characteristic of the metal

- Wave theory could not explain the instantaneous emission of photoelectrons

Front 67

(STM)

A probe is moved horizontally in a straight line across the surface. As it moves, thecurrent due to the transfer of electrons between the surface and the probe decreases thenreturns to its initial value at the end of the line.

Explain why the current changes this way.

Back 67

- As the probe moves along, the gap width increases(as the current decreases) then decreases(as the current increases)

- The current decreases (or increases) because the tunnelling effect(probability of crossing the gap) decreases (or increases)

Front 68

Explain what is meant by the statement that speed of light in free space is invariant

Back 68

C is the same, regardless of the speed of the light source or the observer

Front 69

Explain why the electrons need to be monoenergetic in order for them to emerge incertain directions only

Back 69

- Electrons in beam must have same wavelength

- Otherwise electrons of different wavelengths/speeds/velocities/energies/momenta would diffract by different amounts (for the same order)

Front 70

Millikan made the first accurate measurements of the charge carried by charged oildroplets. Outline what Millikan concluded from these measurements.

Back 70

- The C of each droplet is a whole number of charge of electron

- Concluded that the least amount of charge is the charge of electron

- Quantum charge is 1.6x10^-19 C

Front 71

State one reason why Huygens’ theory of light was rejected for many years after itwas first proposed, in favour of Newton’s corpuscular theory of light.

Back 71

- Newton's other theories were successful

- Huygen's theory was based on longitudinal waves which cannot explain polarisation

Front 72

Explain why the eventual measurement of the speed of light in water led to thedefinite conclusion that light consists of waves and not corpuscles.

Back 72

Either
- Newton predicted that light travels faster in glass than in air, Huygen's predicted the opposite

or

- No evidence (for many years) that light travels slower or faster in glass than in air at the time

- c in water/glass was found later to be less than c in air

Front 73

Newton’s corpuscular theory predicted incorrectly that just two bright fringes would beformed in this pattern. Use Huygens’ theory of light to explain why more than twobright fringes are formed in this pattern.

Back 73

- Pattern is due to interference of light from the two slits

- Interference is a wave property

- Light from two slits is in phase at bright fringe therefore reinforces

- Path difference from central white fringe to the two slits is zero

- Bright fringes are formed away from the centre wherever the path difference is a whole number of wavelengths or dark fringes are formed away from the centre wherever path difference is a whole number of wavelength + half a wavelength

- Path difference for the mth bright fringe from the centre is m wavelengths where m is any whole number

- Since m is any whole number, more than two bright fringes are observed

Front 74

Explain why the beam curves downwards(towards positive plate) at an increasing angle to its initial distance

Back 74

- Force due to electric field acts downwards on electrons

- Vertical components of velocity of each electron increases

- Horizontal component of velocity unchanged (so angle to initial direction increases)

Front 75

Describe, in terms of electric and magnetic fields, a plane polarised electromagnetic wave travelling in a vacuum.

Back 75

- Vibrations of electric wave and magnetic wave

- Perpendicular to each other

- Perpendicular to the direction of propagation

- In phase with each other

Front 76

A detector loop and the transmitter aerial are in the same vertical plane. Explain why an alternating emf is induced in the loop when it is in this position

Back 76

- Magnetic wave vibrations perpendicular to plane of loop

- Magnetic wave causes alternating magnetic flux linkage through the loop

- Alternating magnetic flux induces an alternating emf in the loop

Front 77

Explain why an alternating emf cannot be detected if the detector loop is turned through 90 degrees

Back 77

No magnetic flux linkage through the loop, as loop is now parallel to magnetic wave vibrations so no induced emf

Front 78

State and outline a piece of evidence for the wave nature of light and a piece of evidence for its particle nature. For each piece of evidence, outline a characteristic feature that has been observed or measured and give a short explanation of its relevance to your answer

Back 78

Wave-like property

- Property is interference/diffraction

- Bright and dark fringes of a double slit interference pattern or of the single slit diffraction pattern

- Explanation of bright/dark fringes in terms of path or phase difference

- Particle/corpuscular theory predicts two bright fringes for double slits or a single bright fringe for single slit

Particle-like nature

- Photoelectricity

- Existence of threshold frequency for the incident light/Instant emission of electrons from the metal surface

- E=hf, 1to1 interaction

- Wave theory predicts emission at all light frequencies/delayed emission for low intensity

Front 79

Explain why the interference fringes shift their position if the distance from either of the two mirrors to the semi-silvered block is changed

Back 79

- Bright(or dark) fringe is seen where the two beams are in phase(or out of phase by 180)

- Changing distance to either mirror changes the path (or phase) difference between the two beams so fringes shift

Front 80

Why was it predicted that a shift of the fringes would be observed when the apparatus was rotated through 90 degree?

Back 80

- Speed of light thought to depend on the speed of the light source/motion of the Earth through ether

- Distance travelled by each beam unchanged by rotation

- Time difference between the two beams would change on rotation

- Phase difference would therefore change, so fringes would shift

Front 81

What conclusion was drawn from the observation that the fringes did not shift?

Back 81

- Speed of light is independent of the speed (or motion) of the light source

- Ether hypothesis incorrect

- Absolute motion does not exist

Front 82

State De Broglie's hypothesis

Back 82

- Matter particles have wave-like properties

- And an associated wavelength = h/p where p is the momentum of the particles

Front 83

Describe how Newton used the corpuscular theory to explain the refraction of light as it passes from one substance into a substance of higher optical density

Back 83

- Appreciation that one component changes speed while the other component at right angles does not

- When entering a denser medium a corpuscle accelerates

- There's a short range of attractive force between light corpuscle and the denser material

Front 84

Huygen used a wave theory to explain refraction

Explain why corpuscular theory was rejected in favour of a wave theory to explain refraction

Back 84

- Light travel was shown to travel slower in denser medium

- Newton's theory required light to travel faster, wave theory suggest slower speed

- Newton's theory could not explain slower speed

Front 85

Explain why Michelson and Morley expected that the fringes would shift when the apparatus was rotated 90 degree

Back 85

- They expected the time taken for light to travel in one direction to be different from the other

or

- Expected light to travel at different speeds in the two directions

- There would be a change in phase relationship

Front 86

Discharge tube
Describe how the charged particles responsible for conduction in the gas are produced

Back 86

- Electrons pulled out of as atoms so gas atoms become ions

OR

- Ionisation by collision occurs

OR

- Ions that hit cathode causing it to release electron

- Conduction due to electrons and positive ions

Front 87

Explain why the gas emits light and why it must be at low pressure

Back 87

- Ions and electrons collide with each other and recombine and emit photons

- Electrons excite gas atoms by collision and photons are emitted when de-excitation occurs

- Gas needs to be at sufficiently low pressure in order that the particles in the gas are widely spaced

- Otherwise ions and electrons would be stopped by gas atom OR so that electrons are accelerated to cause excitation

Front 88

The charged particles moving towards the negative electrode were initially referred to as positive rays. Explain why their specific charge depends on the choice of as in the tube

Back 88

- Specific charge = charge/mass (charge of ion does not depend on the type of gas)

- Mass of ion depends on the type of gas

Front 89

A TEM contains magnetic lenses that form an image of a thin sample when a beam of electrons passes through the sample. Explain why a magnetic lens deflects the electrons without changing their kinetic energy

Back 89

- The magnetic force on a moving electron is always perpendicular to its velocity/direction of motion

- No work is done on the electron by the field OR no acceleration in the direction of motion