Photoelectric effect

Question 1

What happens in excitation of atoms?

Answer 1

-Atoms that are in the ground state absorb energy
-As a result one of the electrons from that atom jumps to a higher energy level (more outer one)
-Eventually, the vacancy in the ground state means an electron has to transfer back down to the ground state
-As a result, a photon is given off as the electron transfers back down to the ground state EQUAL to the energy lost by that electron.
ATOMS ABSORB ENERGY BY:
-Excitation by collision
-Excitation by electrons absorbing photons
(ONLY JUMPS When photon energy equal
to an energy level)
REMEMBER: Atoms can de-excite indirectly via SEVERAL energy levels (not just level–>ground), if intermediate levels are present.

Question 2

Describe 4 steps to show how the fluorescent tube works:

Answer 2

Fluorescent tube is a glass tube with a fluorescent coating on its inner surface. Inside contains mercury vapour at low pressure, when it is on it emits light because:

• Ionisation and excitation of mercury atoms occur as they collide with eachother and with electrons
• Mercury atoms therefore GAIN ENERGY from this causing electrons to move to a higher energy level and emit ultraviolet photons as well as visible photons and photons of less energy when they DE-Excite
• The ultraviolet photons are absorbed by the atoms of the fluorescent coating, causing excitation
• These coating atoms de-excite in steps, emitting visible photons

Question 3

The mercury atoms in a fluorescent tube are excited and then emit photons in the
ultraviolet region of the electromagnetic spectrum.
Explain how the mercury atoms become excited.
[3 marks]

Answer 3

-electrons passing through tube collide with electrons in
mercury atom
-transferring energy / atom gains energy from a collision
-causing orbital electrons/electrons in mercury atom to move to
higher energy level

Question 4

Ionisation energy

Answer 4

The amount of energy required to remove an electron from ground state.

Question 5

When light of a certain frequency is shone on a particular metal surface, electrons are emitted
with a range of kinetic energies.
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. (6 marks)

Answer 5

-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 energy of photon exceeds work function
-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 6

Feynmann diagram involving e- + e- —> e- + e-

Name interaction and give exchange particle.

Answer 6

• Electromagnetic

- (Virtual) Photon

Question 7

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

Answer 7

• hf is the amount of energy available

- energy required to remove the electron varies hence kinetic energy of electron varies

Question 8

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

Answer 8

Light energy incident per second/energy of incident photon