The Physics
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# Photoelectric Effect

## Summary

• Electromagnetic radiation can push electrons free from the surface of a solid.
• This process is called the photoelectric effect.
• A material that can exhibit the photoelectric effect is said to be photoemissive.
• Electrons ejected by the photoelectric effect are called photoelectrons.
• The photoelectric effect will not occur when the frequency of the incident light is less than the threshold frequency.
• Different materials have different threshold frequencies.
• Most elements have threshold frequencies in the ultraviolet region of the electromagnetic spectrum.
• The maximum kinetic energy of a stream of photoelectrons…
• is determined by measuring the stopping potential (the applied voltage needed keep the photoelectrons trapped in the photoemissive surface).
• increases linearly with the frequency of the incident light above the threshold frequency.
• is independent of the intensity of the incident light.
• The rate at which photoelectrons are emitted from a photoemissive surface…
• is determined by measuring the electric current.
• is directly proportional to the intensity of the incident light when frequency is constant.
• On a graph of maximum kinetic energy vs. frequency…
• all curves are linear with slope equal to the Planck constant.
• the intercept on the energy axis is the threshold frequency of the material.
• Classical physics cannot explain why…
• no photoelectrons are emitted when the incident light has a frequency below the threshold,
• the maximum kinetic energy of the photoelectrons increases with the frequency of the incident light,
• the maximum kinetic energy of the photoelectrons is independent of the intensity of the incident light, and
• there is essentially no delay between absorption of the radiant energy and the emission of photoelectrons.
• Modern physics states that…
• electromagnetic radiation is composed of discrete entities called photons
• the energy of a photon is proportional to its frequency
• the work function of a material is the energy needed per photon to extract an electron from its surface
• Equations
• photoelectric effect

Kmax = E − ϕ = h(f − f0)

• photon energy  E = hf = hc λ
• work function  ϕ = hf0 = hc λ0
• stopping potential

Kmax = eV0