Explanation Physics

Stationary waves on a guitar

• Waves are produced and travel to the boundaries and are reflected towards each other

• Two waves travelling in opposite directions interfere/superpose with each other

• This produces fixed boundaries of zero amplitude called nodes

• In some positions the waves always cancel/ interfere destructively to give zero amplitude/no vibration/nodes

• In some positions the waves interfere constructively to produce positions of maximum amplitude/max vibration/antinodes

Photon explanation of threshold frequency

• According to classical theory, the threshold frequency shouldn't exist -> if light was a wave, low energy intense light should be able to release electrons

• But the emission of only electrons depends on the frequency (energy) of the light as the energy of a photon is E =hf

• There is an one to one interaction between a single incident photon and a single electron

Gold leaf Electroscope

• If we being a UV light source there will be emission of electrons from the surface

• Number of surplus electrons that remain on the surface will decrease with time → i.e. the negative charge on the plate will decrease, meaning that the gold leaf would fall

This happens because:

• There is a minimum amount of energy required to release an electron and photons must supply this energy in one interaction

• UV photons can do that as their energy E=hf is greater than the work function

Stopping potential - Photocells example

Photocells demonstrate the photoelectric effect:

• If photons of energy higher than the work function of the photoemissive surfaces are incident electrons will be released

• These electrons are then detected as current by the ammeter in the circuit

If the intensity of the incident radiation is increased:

• More photons per second are incident of the surface

• More electrons are released per second from the surface

• the current in the circuit increases as it is the rate of flow of charge

Ionisation and Excitation in a mercury fluorescent tube

• Electrons passing through the tube collide with electrons in the mercury atoms

• They transfer energy and the mercury electrons go into higher energy levels

• Excited electrons then de-excite to lower energy levels

• Emitting a photon of equal energy difference to the energy difference between the energy levels

• The emitted photon is in the UV range

• The coating of the mercury tube absorbs UV photons and electrons in the coating are excited to higher energy levels

• Atomic electrons de-excite to a previous lower level and emit lower energy photons

Energy levels

• When electrons absorb a photon, they go up in energy level and excite

• When electrons de-excite and go down in energy they release a photon, equal to the energy level difference

Why are energy levels negative

• Energy levels are negative as to remove an electron, energy must be supplied

• Once free an electron has zero energy

Wave particle duality

• Photoelectric effect suggests that electromagnetic waves have a particle nature

• Electron diffraction suggests that particles have a wave nature

Particle behaviour

• Pure particle behaviour would produce a pure spot of light with some particles scattered randomly

• However the pattern seen shows diffraction, which is a pure wave property

• Bright rings occur where constructive interference occurs, similar to the diffraction grating formula

What happens when V increases? (Electron diffraction experiment)

• If pd increases (Accelerating p.d) the momentum of the electrons increases

• So their De Broglie wavelength decreases

• I.e. The diameter of the rings pattern will decrease

Line emission spectrum

• A set of discrete bright lines on a dark background, produced when excited atoms emit photons as electrons drop to lower energy levels → photon energies correspond to downward transitions.

• Line spectra exist because atomic energy levels are discrete

• Emission and absorption line positions match for the same gas (same energy gaps)

How they form:

• If a low-pressure gas is excited (e.g. in a discharge tube), electrons in atoms can move to higher energy levels

• When they return to lower levels, they emit photons with energies equal to the differences between levels

• This is because only certain energy differences exist, only certain photon frequencies/wavelengths appear.

Line absorption spectra

• A continuous spectrum with discrete dark lines removed, produced when atoms absorb photons of specific energies that match allowed transitions → only photons with exactly the right energies are absorbed (matching energy gaps)

• Emission and absorption line positions match for the same gas (same energy gaps)

How they form:

• If white light passes through a cooler gas, atoms absorb photons whose energies match allowed upward transitions

• Those wavelengths are missing from the transmitted light, so dark lines appear

Explain why a dark fringe produced

phase difference is 180° or π / path difference is λ2 / anti phase

Destructive interference

Explain why some waves can be polarized but others cannot

• Transverse waves oscillations are perpendicular to direction of energy transfer

• Whilst longitudinal waves oscillation are parallel to the direction of energy transfer

• Polarisation is restriction of displace vector to one plane

Explain how the minima of intensity occur

• Two waves superpose/interfere with each other when they move in opposite directions towards each other

• This means for minima to be observed, they are permanently in anti-phase with each other

Explain how electron diffraction shows evidence for wave nature of electrons

• Particle behaviour would only produce a patch/circle of light/small spot of light

• Wave property shown by diffraction or interference

• Graphite causes electron waves to spread out or electrons to travel in particular directions

• Bright rings/maximum intensity occurs where waves interfere constructively/ are in phase

State and explain what happens to the value of angle θ for the first order beam if the wavelength of the monochromatic light decreases

• angle θ gets smaller (1)

• because path difference gets smaller/d constant, (λ smaller) so sin θ smaller

Describe what the observer would see as Q is rotated slowly through 360

• Variation in intensity between max and min (or light and dark) (1)

• Two maxima (or two minima) in 360° rotation (1)

Explain briefly how energy is transmitted in this sound wave

• Particles in the transmitting medium are made to vibrate/given energy

• Compression/region of increased pressure (or rarefaction) causes nearby particles to vibrate/have energy/move