The Photoelectric Effect

The Photoelectric Effect

Shining a light on a metal releases electrons

What does whether the photoelectric effect happens depend on

The frequency of light

Which part of the EM spectrum does the threshold frequency does the light causing the photoelectric effect fall in

UV

Describe the photoelectric effect

Free electrons on the surface of metal absorb energy from light, causing the bonds holding it to the metal to break, releasing it

Photoelectron

An electron released by the photoelectric effect

Photoelectric effect conclusion 1

For a given metal, no photoelectrons are emitted if the radiation has a frequency below the threshold frequency

Photoelectric effect conclusion 2

Photoelectrons are emitted with a range of kinetic energies ranging from zero to some maximum value, increasing with the frequency of radiation, but unaffected by intensity

Photoelectric effect conclusion 3

Number of photoelectrons emitted per second is proportional to the intensity of radiation

Intensity according to wave theory

For a particular frequency of light, the energy carried is proportional to the intensity of the beam

According to wave theory how is energy spread in a wave

Evenly over the wavefront

Why can’t the photoelectric effect be explained by wave theory

Electron energy should increase with intensity, not with frequency

Why doesn’t threshold frequency work with wave theory

No matter the frequency, after enough time. electrons should be emitted eventually

Photoelectric effect according to the photon model

When light hits a surface, the metal is bombarded by photons, if one is absorbed by a free electron, it will gain energy equal to hf

ϕ symbol

Work function energy

Work function energy

The energy an electron needs to break the bonds holding it to the surface of the metal

What does ϕ depend on

The metal

What is the threshold frequency

f = ϕ/h

Why does the photon model explain threshold frequency

Electrons will only be emitted if the energy gained from a photon is greater than the work function

How is the range of kinetic energies explained

Electrons deeper in the metal lose more energy than electrons on the surface

How is the photoelectric equation derived

The minimum amount of energy an electron can lose is the work function, so the maximum kinetic energy = hf - ϕ, so hf = ϕ + maximum kinetic energy

Photoelectric equation

hf = ϕ + 1/2mv²_max

Why is the kinetic energy of an electron in the photoelectric effect independent of intensity

Intensity is the number of photons per second on an area, but electrons can only absorb one photon at a time