The Photoelectric effect and the Photon Model

Question 1

High Frequency Light can…

Answer 1

…Release electrons from the surface of a metal.
- Frequency that this occurs for most metals falls around the UV light range.

Photoelectric effect:
- Free electrons on the surface of a metal can sometimes absorb energy from the light hitting the surface.
- If the electrons absorb enough energy, the bonds holding them to the metal can be broken and the electron is released.
- These electrons emitted are called photoelectrons.

Question 2

3 Main conclusions of the Photoelectric effect?

1. For a given metal…
2. The photoelectrons are emitted with…
3. The number of photoelectrons emitted per second is…

Answer 2

1. For a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value - this is called the threshold frequency.
2. The photoelectrons are emitted with a variety of different kinetic energies, ranging from 0 to a max value - the value of max EK increases with the frequency of the radiation, intensity of the radiation has no effect on the kinetic energy.
3. The number of photoelectrons emitted per second is directly proportional to the intensity of radiation

Wave Theory cannot explain the photoelectric effect: according to wave theory, electrons should be emitted eventually, no matter what the frequency is.

Question 3

Ultraviolet Catastrophe

Explain

Answer 3

A black body is a perfect absorber and emitter .
Black Body definition: A body that absorbs all wavelengths of EM radiation due to their temperature.
- Wave theory could not explain the graph of Intensity against Wavelength for black body radiation. Could only explain slope of graph at long wavelength (low frequencies). But predicted an infinitley high peak towards the UV range

Question 4

Photon Model of light

Background

Answer 4

When Max Planck was investigating black body radiation he suggested that EM waves could on be released in discrete packets, or quanta.
Energy carried by one of these wave packets had to be: E = hf = hc / λ
Einstein went further and suggested that EM waves and the energy they carry can only exist in discrete packets. He called these wave-packets, Photons.
He saw these photons of light having a one-on-one, particle like interactions.

Question 5

Photon Model of light

Summary

Answer 5

• When light hits a metal suface, the metal is bombarded by photons.
• If one of these photons collides with a free electron in the metal, the free electron will gain energy equal to hf. If hf is greater than the work function, the electron can be emitted.
• Each electron only absorbs a single photon at a time, therefore all the energy the electron needs to surpass the work function and be emitted needs to come from one photon.