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 (f0).
• 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 (Kmax)…
• is determined by measuring the stopping potential (V0) the applied voltage needed keep the photoelectrons trapped in the photoemissive surface

Kmax = eV0

• 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 Planck's constant

h = 6.63 × 10−34 J s

• the intercept on the energy axis is the threshold frequency of the material
• Magnify

• 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
• 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
E ∝ f  ⇒   E = hf = hc λ
• the work function (φ) of a material is the energy needed per photon to extract an electron from its surface  φ = hf0 = hc λ0
• From conservation of energy then…

Kmax = E − φ = h(f − f0)