EM Radiation & Quantum Phenomena

Question 1

Describe the photoelectric effect.

Answer 1

Photons of EM radiation are incident on a metal surface.

1 photon transfers all its energy to 1 electron near the surface.

Photoelectrons emitted.

Question 2

Give details about the kinetic energy of the photoelectrons emitted.

Answer 2

Photoelectrons have a range of kinetic energies,

From 0 up to a maximum value (determined by frequency of radiation).

Question 3

Define work function (φ).

Answer 3

The minimum photon energy required,

to cause the emission of an electron from the surface of a metal.

Question 4

Define threshold frequency (f₀).

Answer 4

The minimum frequency of EM radiation required,

to cause the emission of an electron from the surface of a metal.

Question 5

Give the equation linking work function and threshold frequency.

Answer 5

As energy of the photon ≥

work function and E= hf:

φ = hf₀

Question 6

Give the equation linking frequency, work function and max kinetic energy of electrons.

Answer 6

As photon energy = work function + max KE of electrons:

hf = φ + E_k(max)

Question 7

Explain how increasing frequency (or decreasing wavelength) of EM radiation affects the photoelectric effect.

Answer 7

If frequency (f) increases or wavelength (ʎ) decreases,

Photon energy (E) increases (as E=hf or E = hc/ʎ),

Work function (φ) is constant,

Maximum kinetic energy of electrons increases as E_k(max) = hf – φ.

Question 8

Explain how increasing intensity (power per unit area) of EM radiation affects the photoelectric effect.

Answer 8

If intensity increases, the number of photons incident per second increases.

As 1 photon transfers all its energy to 1 electron.

Number of photoelectrons emitted per second increases.

Question 9

Define stopping potential

Answer 9

The potential difference required,

to stop the emission of electrons from the surface of a metal.

Question 10

Give the equation for stopping potential.

Answer 10

As work done stopping fastest electrons = max KE of electrons,

V_s = E_k(max) / e

Question 11

Define ground state.

Answer 11

Lowest energy level (n=1).

Closest to the nucleus.

Question 12

Define excitation in terms of energy levels and energy.

Answer 12

An electron moves to a higher energy level,

when it gains energy,

equal to the difference between the two levels.

Question 13

Describe two ways that electrons can gain energy to become excited.

Answer 13

1 photon transfers all its energy to 1 electron.

A free electron collides with the electron and transfers some energy from its kinetic store.

Question 14

Define de-excitation in terms of energy levels, energy and photons.

Answer 14

An electron moves to a lower energy level,

when it loses energy,

by emitting a photon,

with energy equal to the difference between the two levels.

Can happen directly or in stages/cascading.

Question 15

Define ionisation.

Answer 15

An electron gains enough energy,

to be removed from an atom.

Question 16

Define ionisation energy.

Answer 16

The amount of energy required,

to completely remove an electron from an atom,

from the ground state.

Question 17

Why are energy levels defined as having negative energy?

Answer 17

A free electron removed from an atom is defined as 0 J.

All energy levels are negative relative to this, as energy needs to be supplied to remove an electron.

Question 18

Describe the structure of the discharge tube used for fluorescent lights.

Answer 18

Glass tube filled with mercury vapour.

Beam of free electrons accelerated by a p.d..

Phosphor coating.

Question 19

Describe what happens to the electrons in the mercury vapour in the flourescent light.

Answer 19

Free electrons collide with electrons in mercury atoms and transfer energy.

Mercury electrons excite then immediately de-excite,

emitting UV photons.

Question 20

Describe what happens to the electrons in the phosphor coating in flourescent lights.

Answer 20

UV photons are absorbed by electrons in phosphor coating.

Phosphor electrons are excited,

then de-excite in stages, emitting visible light photons.

Question 21

What do line emission spectra look like?

Answer 21

Black background.

Coloured lines at certain wavelengths.

Question 22

How are line emission spectra formed?

Answer 22

Discharge tube filled with gas.

Electrons in the gas are excited when free electrons collide and transfer energy.

De-excitation of electrons causes photons to be emitted with certain energies equal to difference in energy levels,

which correspond to certain wavelength / frequencies,

as E=hf or E = hc/ʎ.

Question 23

What do line absorption spectra look like?

Answer 23

Colour spectrum background.

Black lines at certain wavelengths.

Question 24

How are line absorption spectra formed?

Answer 24

White light is shone through a cold gas.

Electrons are excited when they absorb photons with certain energies,

equal to the difference in energy levels,

which correspond to certain wavelength / frequencies,

as E=hf or E = hc/ʎ.

Question 25

What is meant by the duality of particles?

Answer 25

Particles can behave like particles and like waves.

Question 26

What is the equation for de Broglie wavelength? (Data sheet)

Answer 26

ʎ = h/p = h/mv

Question 27

What is diffraction?

Answer 27

Spreading out of waves,

as they pass through a gap /go round an obstacle.

Question 28

When does diffraction happen?

Answer 28

When the wavelength is the same order of magnitude as gap size / obstacle size.

Question 29

What factors affect diffraction?

Answer 29

Smaller gap -> greater diffraction effect.

Longer wavelength -> greater diffraction effect.

Question 30

In electron diffraction, how do electrons reach high speeds? Is this particle or wave behaviour?

Answer 30

Accelerated by a potential difference.

Particle behaviour.

Question 31

In electron diffraction, how are electrons diffracted? Is this particle or wave behaviour?

Answer 31

Diffracted through the gaps between atoms in graphite

Wave behaviour.

Question 32

In electron diffraction, how do the electrons produce the pattern on the screen? Is this particle or wave behaviour?

Answer 32

Electrons collide with electrons in the screen causing fluorescence.

Particle behaviour.