Y1: Electromagnetic Radiation and Quantum Phenomena

Question 1

What is the photoelectric effect

Answer 1

If you shine radiation of a high enough frequency onto the surface of a metal, it will emit electrons

Question 2

What causes the photoelectric effect

Answer 2

Free electrons near the surface of the metal absorb energy from the oncoming radiation, causing them to vibrate. If enough energy is absorbed, the bond between the metal and the electron in overcome, releasing it as a photoelectron.

Question 3

What are the conclusions from the photoelectric effect experiments

Answer 3

• No photoelectrons can ever be emitted if the radiation freq. is below the threshold freq.
• Photoelectrons are emitted with a variety of kinetic energies, with the max value increasing at higher frequencies (Ek not affected by radiation intensity)
• The No. of photoelectrons emitted per second is proportional to the radiation intensity

Question 4

What is the photon model of light

Answer 4

EM waves can only exist as discrete packets, carrying a certain energy (E=hf), and these photons have 1:1 interactions with the electrons as part of the photoelectric effect.

Question 5

What would be the expected observation if the frequency of the radiation is below the threshold frequency, according to the wave theory.

Answer 5

If the light interacted with the electrons as a wave, it would take longer for the photoelectrons to be emitted with low freq. radiation, but they would eventually, as their energy would gradually increase

However, this is not observed

Question 6

How does the threshold frequency support the photon model of light.

Answer 6

The energy of a photon is proportional to the freq. (E=hf), and each photon only contains a discrete quantity of energy. This means that if the freq. of the radiation is too low, the interaction with the electron will never cause it to be emitted, as there is not enough energy to overcome the bond with the metal.

Question 7

What would be the expected observation if the intensity of the radiation increased, according to the wave theory.

Answer 7

If the light interacted with the electrons as a wave, higher intensity would increase the energy transferred to each electron, increasing their Kinetic energy

However, this is not observed

Question 8

How does the kinetic energy of photoelectrons support the photon model of light.

Answer 8

Increasing the intensity of the radiation causes an increase in the number of photons, but doesn’t effect the energy of each one. This means that each interaction occurs with the same energy (proportional to freq.), so the electrons have the same Ek, but there are more interactions, so the total number of photoelectrons released increases.

Question 9

What is the gold foil experiment for demonstrating the photoelectric effect.

Answer 9

• A zinc plate is attached to the top of an electroscope (box containing a piece of metal with gold leaf attached).
• The zinc plate is negatively charged, so the metal and the gold repel each other (gold foil lifts up)
• UV light (high freq. - Red light wouldn’t work) is shone on the zinc plate
• The energy of the photons causes electrons to be released (photoelectric effect)
• This causes the zinc (and metal) to lose their charge, so the gold leaf is no longer repelled and falls back down.

Question 10

What is the work function (ɸ)

Answer 10

The energy required to break the bond holding the electron to the metal

Question 11

What is the equation for work function (ɸ)

Answer 11

In order for the photoelectric effect to occur

hf ≥ ɸ

Question 12

What is the threshold frequency (fo)

Answer 12

The minimum frequency of a photon (radiation) needed for the photoelectric effect to occur

Question 13

What is the equation for the threshold frequency (fo)

Answer 13

fo = ɸ/h

Question 14

How do you calculate the maximum kinetic energy of a photoelectron (Ek)

Answer 14

hf = ɸ + Ek(max)

Question 15

What is the stopping potential (Vs)

Answer 15

The potential difference required to stop the fastest (Ek max) moving electrons, as they lose their energy doing work against it.

Question 16

What is the equation for stopping potential

Answer 16

eVs = Ek(max)

(e = 1.6x10^-19C)

Question 17

What is an electron volt (eV)

Answer 17

The kinetic energy carried by an electron after it has been accelerated from rest to a pd of 1V

Question 18

What is 1eV

Answer 18

1.6x10^-19J

Question 19

What is a ground state electron

Answer 19

An electron in the lowest energy level in an atom (n=1)

Question 20

How does an electron move up energy levels

Answer 20

If it absorbs a photon with the exact energy of the difference between the two levels, the electron is excited.

Question 21

How does an electron move down energy levels

Answer 21

If it emits a photon with the exact energy of the difference between the two energy levels

Question 22

Why are the energies of the different energy levels in an atom negative

Answer 22

The electrons within this level are bound to the atom (like a ball in a pit with -GPE), and at infinity, the energy is 0J, so levels closer to the atom have negative values.

Question 23

What is the ionisation energy

Answer 23

The energy required to remove and electron from an atom, beginning at ground state.

Question 24

How do you work out the ionisation energy of an atom

Answer 24

The energy of an electron must reach 0J to escape the atom, so the ionisation energy is the positive value of the energy at ground state (eg. Hydrogen = 13.6eV)

Question 25

How are photons (visible light) emitted by fluorescent tubes

Answer 25

• Tube contain mercury vapour, with a high voltage supplied across it.
• This accelerates free electrons, ionising some mercury atoms to produce more free electrons
• Free electrons collide with electrons in mercury atoms, exciting them to higher energy levels.
• As the electrons return to ground state, they emit high energy photons (UV).
• These photons are absorbed by phosphor coating on the inside of the tube, exciting it’s electrons
• When these electrons move down energy levels, they emit many lower energy photons of visible light.

Question 26

What is line emission spectra

Answer 26

When photons (light) are emitted from an atom as it’s electrons de-excite, the light can be diffracted to separate out the wavelengths.
This will show a black screen with several coloured lines corresponding to the wavelengths of the photons emitted.

Question 27

What is line absorption spectra

Answer 27

When white light passes through a cool gas, certain wavelengths of photons are absorbed as the electrons are excited.
The remaining light that passes through can be diffracted to give a colour spectrum, with several dark lines corresponding to the wavelengths of photons that were absorbed.

Question 28

Why must the light pass through a cool gas for line absorption spectra

Answer 28

At low temperatures, most of the electrons will be at ground state, so all possible photons that can be absorbed, will be.

Question 29

How can line emission/absorption spectra be used to determine the elements within a gas/object in deep space.

Answer 29

The lines on the spectra correspond with the wavelengths of photons emitted/absorbed by the atoms, and as E∝λ (E=cf/λ), the wavelengths correspond to the energies of the photons, which is exactly equal to the difference in the energy levels of the atoms. This means that the line emission/absorption spectra is unique for each element.

Question 30

When does light have wave-like properties

Answer 30

Diffraction: When a beam of light passes through a narrow gap, it spreads out, which can only be explained if light acts as a wave.

Question 31

When does light have particle like properties

Answer 31

Photoelectric effect: When photons hit a metal, electrons are released, and increasing the light intensity only increases the number of electrons (not the Ek of each), which can only be explained is light is considered as particle-like photons.

Question 32

What is the wave-particle duality theory

Answer 32

If ‘wave-like’ light shows particle properties (photons), particles such as electrons should show ‘wave-like’ properties.

Question 33

What is the de Broglie wavelength equation

Answer 33

λ = h/mv

Links a ‘wave-like’ property (λ) with a particle property (momentum)

Question 34

How does electron diffraction occur

Answer 34

Electrons are accelerated to high velocities in a diffraction tube, then passed through a graphite crystal.
This produces a diffraction pattern of rings (of the probability of where an electron will end up).

Question 35

How does changing the speed of the electrons in the diffraction tube effect the diffraction pattern

Answer 35

λ ∝ diffraction spacing (just like in light),
and as λ=h/mv: Diffraction spacing ∝ 1/v

So, slower electrons form widely spaced rings, and vice versa

Question 36

How does changing the mass of the particles in the diffraction tube (eg. neutrons instead of electrons) effect the diffraction pattern

Answer 36

λ ∝ diffraction spacing (just like in light),
and as λ=h/mv: Diffraction spacing ∝ 1/m

So, heavier particles form more tightly spaced rings, and vice versa

Question 37

What must the size of the lattice spacing be in a crystalline structure for diffraction to occur through it

Answer 37

The lattice spacing must be similar to the wavelength of the incident radiation in order for diffraction to occur through a crystalline structure.