Quantum - Photoelectric Effect

Question 1

Photoelectric Effect

Answer 1

• The photoelectric effect is a phenomenon in which, when light shines on a metal surface, electrons are ejected from it.
• It provides important evidence that light is quantised or carried in discrete packets.

• Experiments in the late 19th Century led to some laws about the effect:
‣ Electrons are only emitted if the frequency
of the light is above a certain frequency
‣ If the frequency is high enough, the
the intensity of the light governs how many
electrons are emitted
‣ The maximum Kinetic energy of emitted
electrons is independent of the intensity of
the light, but proportional to the frequency
of the light

Question 2

Classical Predictions

Answer 2

• Electrons require some energy to escape from the surface of the metal
• The minimum energy required to escape from the surface of the metal is known as the work function
• According to the wave theory of light, the energy of the incident light is spread over the whole surface. Electrons should therefore only be emitted if the incident light is given by:
Intensity = power absorbed/Area is high enough.

Question 3

Quantisation

Answer 3

Einstein explained the experimental evidence by assuming that light comes in discrete quanta or packets of energy, which became known as photons.

Each photon has a specific energy E=hf and only collides with one electron, as the probability of two photons colliding with one electron is very low. Thus an electron can only absorb a specified amount of energy for the light of a specific frequency.

If this energy is insufficient to allow an electron to escape the surface of the metal, no electrons will be emitted. This creates a threshold frequency, above which a photon provides an electron with enough energy to leave the surface, and below which the electrons cannot escape. The minimum energy required to remove an electron from the metal is the work function ϕ. If a photon provides more energy to the electron than this, the rest will be seen as kinetic energy.

Increasing the intensity affects the rate of electrons being emitted, but does not affect their maximum kinetic energy. This is because increasing the intensity (at the same frequency) increases the number of photons hitting the metal per second, and the number of electrons that absorb them, but has no effect on the energy carried by each photon. Below the threshold frequency, even if a huge number of electrons absorb photons, none of them has enough energy to escape the surface.