The Photoelectric Effect

What is a photon?

One particle of light

Explain the photoelectric effect.

• If light is shone at a metal surface, electrons with either leave the surface or not

• If the light is at a high frequency, electrons leave the surface but not at low frequency

Why do electrons only leave the surface of the metal at a high frequency?

Because the frequency determines the energy of each photon, and electrons can only be emitted if the absorb enough energy from the photons

How does the photoelectric effect prove that light can act as particles?

If light were to behave as a wave, increasing the intensity or length of time that the light shines for would allow the electrons to absorb enough energy to be liberated. However, this is not the case, showing that there are one-to-one interactions between the light particles and the electrons.

Define threshold frequency.

The minimum frequency of light that allows electrons to leave the surface of the metal

State the wave equation.

Speed (of light) = frequency x wavelength

State Planck’s equation.

Energy = (6.63 × 10^-34) x frequency —> E = hf

Define work function.

The minimum photon energy for an electron to be liberated from the metal surface

How many joules are in 1 electron volt?

1.6 × 10^-19

What is wave-particle duality?

The idea that things can behave as either waves OR particles

Give an example of light behaving as a wave.

When light rays are incident on a narrow gap, they will diffract (a wave behaviour)

Give an example of matter behaving as a particle.

When electrons pass through a magnetic or electric field, they deflect (a particle behaviour)

Give an example of light behaving as a particle.

The photoelectric effect, where there are clear one-to-one interactions between photons and electrons

Give an example of matter behaving as a wave.

When electrons are incident on narrow slits, they will diffract and cause an interference pattern (a wave behaviour)

What happens when low energy, low intensity light is shone at a metal surface?

No electrons are emitted as the energy is below the work function

What happens when low energy, high intensity light is shone at a metal surface?

No electrons are emitted as photon energy is below the work function

What happens when high energy, low intensity light is shone at a metal surface?

Some electrons leave the surface, as photon energy is above the work function

What happens when high energy, high intensity light is shone at a metal surface?

Lots of electrons leave the surface

How does the intensity of the light effect the release of electrons?

At high intensity, more electrons will be emitted per second as there are more photons hitting the surface per second

How does the frequency (above the work function) effect the electrons that are released?

They will have more kinetic energy after liberation because the photons have more energy, and the electrons only use some of this to leave the metal, so the rest is used as kinetic energy

How do you calculate the maximum kinetic energy of an emitted electron?

Kinetic energy = hf - work func.

Why is the calculated kinetic energy of an emitted electron described as the maximum?

Because some electrons that are deeper in the metal surface may need more energy (than the work function) to be liberated, so will have less remaining to use as kinetic energy

Explain how a vacuum photocell works.

1. Light is incident on a metal plate (the photocathode)

2. If the frequency of the light is above the threshold frequency, electrons are emitted

3. They travel across the gap to the anode and produce a current in the circuit

Explain the term stopping potential.

If the potential at the anode is negative enough, the electrons emitted by the photocathode can be turned back (repelled) so that they don’t reach the anode and no current flows.

The stopping potential is the potential at which the current stops flowing

How do you calculate stopping potential?

e x V(Stop) = ½ mv² —> e = 1.6 × 10^-19

What is the deBroglie wavelength?

The wavelength of a particle when it acts as a wave

How do you calculate deBroglie wavelength?

DB wavelength = h / (mass x velocity)

What is the mass of an electron?

9.11 × 10^-31