Quantum physics - photoelectricity

Question 1

When a clean metal surface in a vacuum is irradiated with ultraviolet radiation of a certain
frequency, electrons are emitted from the metal.
(a) (i) Explain why the kinetic energy of the emitted electrons has a maximum value (2)

Answer 1

hf is energy available/received or same energy from photons (1)
energy required to remove the electron varies (hence kinetic
energy of electrons will vary) (1)

Question 2

Explain with reference to the work function why, if the frequency of the radiation is
below a certain value, electrons are not emitted. (2)

Answer 2

work function is the minimum energy needed to release
an electron (1)
Energy of the photon depends on its frequency E=hf. Energy of the photon has to be greater than work function for electrons to be emitted (1)

Question 3

State a unit for work function (1)

Answer 3

joule (1)

Question 4

Light energy is incident on each square millimetre of the surface at a rate of
3.0 × 10–10 J s–1
. The frequency of the light is 1.5 × 1015 Hz.
(i) Calculate the energy of an incident photon. (2)

Answer 4

(use of E = hf)
energy = 6.63 × 10–34 × 1.5 × 1015 (1)
energy = 9.9 × 10–19 (J) (1)

Question 5

Calculate the number of photons incident per second on each square millimetre of
the metal surface (2)

Answer 5

number of photons per second = 3.0 × 10–10/9.9 × 10–19 (1)
number of photons per second = 3.0 × 108
(1)

Question 6

In the wave theory model of light, electrons on the surface of a metal absorb energy from a
small area of the surface.
(i) The light striking the surface delivers energy to this small area at a rate of
3.0 × 10–22 J s–1
.
The minimum energy required to liberate the electron is 6.8 × 10–19 J.
Calculate the minimum time it would take an electron to absorb this amount of
energy.

Answer 6

(time taken = 6.8 × 10–19/3 × 10–22)
time taken = 2.3 × 103
s (1)

Question 7

In practice the time delay calculated in part c (i) does not occur. Explain how this
experimental evidence was used to develop the particle model for the behaviour of
light. (1)

Answer 7

light travels as particles/ photons (1)

or has a particle(like) nature

Question 8

When light of a certain frequency is shone on a particular metal surface, electrons are
emitted with a range of kinetic energies.
(a) Explain
• in terms of photons why electrons are released from the metal surface, and
• why the kinetic energy of the emitted electrons varies upto a maximum value

Answer 8

• energy is needed to remove an electron from the surface
• work function φ (of the metal) is the minimum energy needed by an electron to escape from the surface
• light consists of photons , each of energy E = hf
• one photon is absorbed by one electron
• an electron can escape (from the surface) if hf > φ
• kinetic energy of an emitted electron cannot be greater than hf – φ
• an electron below the surface needs to do work/uses energy to reach the surface
• kinetic energy of such an electron will be less than hf – φ

Question 9

The photoelectric effect suggests that electromagnetic waves can exhibit particle-like
behaviour. Explain what is meant by threshold frequency and why the existence of a
threshold frequency supports the particle nature of electromagnetic waves. (6)

Answer 9

• threshold frequency minimum frequency for emission of electrons
• if frequency below the threshold frequency, no emission
even if intensity increased
• because the energy of the photon is less than the work function
• wave theory can not explain this as states that the energy of wave
increases with intensity
• light must travel as photons
• as photons have energy that depends on frequency
• if frequency the is above the threshold, the photons enough energy to emit electrons
• also there is no time delay when the photon emitter is turned on and when the electrons are emitted suggesting a 1-1 relationship between photon and electron

Question 10

when light is shone on a surface the photoelectric effect takes place. Explain the effect on the emitted electrons when the frequency of the light is increased (2)

Answer 10

the (maximum) kinetic energy/speed/velocity/momentum
of released electrons increases (1)

this is because increasing the frequency of the photons increases
their energy (1)

Question 11

Explain the effect on the emitted electrons when the intensity of the light is increased (2)

Answer 11

the number of electrons emitted (per second) increases (1)

because there are now more photons striking the metal surface (1)

Question 12

explain what is needed to happen for a theory to be validated (2)

Answer 12

experiment/observation needs to be performed (to test a theory) (1)
the results of (the experiment) need to be proved/repeatable/replicated/
confirmed by other scientists (1)

Question 13

State what is meant by work function. (2)

Answer 13

minimum energy required

to remove electron from metal (surface) (2)

Question 14

Explain why the emitted electrons have a range of kinetic energies up to a maximum
value. (4)

Answer 14

energy of photons constant (1)
one to one interaction between photon and electron (1)
Max KE = photon energy – work function (1)
more energy required to remove deeper electrons (1)

Question 15

In a fluorescent tube, how does the mercury vapour and the coating of its inner surface contribute to the production of visible light. (6)

Answer 15

mercury vapour at low pressure is conducting (1)
atoms of mercury are excited by electron impact (1)
producing (mainly) ultra violet radiation (1)
which is absorbed/ excites the coating (1)
which, upon relaxing, produces visible light (1)
electrons cascade down energy levels (1)

Question 16

what is meant by the ionisation of an atom. (1)

Answer 16

when an atom loses an orbiting electron (and becomes charged) (1)

Question 17

what is meant by ground state (1)

Answer 17

when electrons lowest energy

Question 18

Explain the difference between excitation and ionisation. (3)

Answer 18

•in either case an electron receives energy
•excitation promotes an electron to a higher energy level
•whilst ionisation occurs to leave
the atom

Question 19

An atom can also become excited by the absorption of photons. why can only photons
of certain frequencies cause excitation in a particular atom (4)

Answer 19

electrons occupy discrete energy levels (1)
and need to absorb an exact amount of energy to move to a higher level (1)
photons need to have certain frequency to provide this energy (1)
all energy of the photon is absorbed (1)
in 1 to 1 interaction (1)

Question 20

What is meant by an excited atom? (1)

Answer 20

electron moved up to a higher energy level (1)

Question 21

Describe the process by which mercury atoms become excited in a fluorescent tube. (3)

Answer 21

electrons flow through the tube (1)
and collide with orbiting electrons or mercury atoms (1)
raising the electrons to a higher level in the mercury atoms (1)

Question 22

What is the purpose of the coating on the inside surface of the glass in a fluorescent
tube? (3)

Answer 22

photons emitted from mercury atoms are in the ultra
violet spectrum (1)
these photons are absorbed by the powder (1)
and the powder emits photons in the visible spectrum (1)

Question 23

State what kind of experiment would confirm that electrons have a wave-like nature. (1)

Answer 23

diffraction experiments (1)

Question 24

State why it is easier to demonstrate the wave properties of electrons than todemonstrate wave properties of protons. (1)

Answer 24

easier to obtain electrons (1)