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
- is determined by measuring the stopping potential (V0) the applied voltage needed keep the photoelectrons trapped in the photoemissive surface
- 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
- all curves are linear with slope equal to Planck's constant
- 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)