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Physics
The Photoelectric Effect
Light, Photons, and Electron Emission
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The photoelectric effect occurs when light strikes a metal surface and electrons are emitted from that surface. This effect matters because it showed that light can behave like particles called photons, not only like waves. Classical wave theory could not explain why very dim high-frequency light can eject electrons while bright low-frequency light cannot. Einstein explained the effect by connecting each photon’s energy to its frequency.
Key Facts
- Photon energy is E = hf, where h is Planck’s constant and f is frequency.
- Electrons are emitted only if hf >= φ, where φ is the work function of the metal.
- Maximum kinetic energy is Kmax = hf - φ.
- The stopping potential satisfies eVs = Kmax.
- Increasing light intensity increases the number of emitted electrons if f is above the threshold frequency.
- Threshold frequency is f0 = φ/h, the minimum frequency needed to eject electrons.
Vocabulary
- Photon
- A photon is a particle-like packet of electromagnetic radiation with energy E = hf.
- Work function
- The work function is the minimum energy needed to remove an electron from a metal surface.
- Threshold frequency
- The threshold frequency is the lowest light frequency that can eject electrons from a particular metal.
- Photoelectron
- A photoelectron is an electron emitted from a material after absorbing energy from a photon.
- Stopping potential
- The stopping potential is the voltage needed to reduce the maximum photoelectron current to zero.
Common Mistakes to Avoid
- Thinking brighter light always ejects electrons is wrong because intensity cannot overcome a frequency below the threshold frequency.
- Using wavelength directly in E = hf is wrong because the formula needs frequency, so convert with c = fλ when wavelength is given.
- Forgetting the work function is wrong because not all photon energy becomes kinetic energy of the electron.
- Assuming emitted electrons all have the same kinetic energy is wrong because electrons start at different depths and binding conditions, so Kmax describes only the fastest emitted electrons.
Practice Questions
- 1 A photon has frequency 8.0 x 10^14 Hz. Using h = 6.63 x 10^-34 J s, calculate its energy in joules.
- 2 A metal has work function 2.3 eV and is struck by photons of energy 3.8 eV. What is the maximum kinetic energy of the emitted electrons in eV, and what stopping potential is required?
- 3 A metal does not emit electrons when exposed to intense red light, but it does emit electrons when exposed to weak ultraviolet light. Explain what this shows about frequency, intensity, and the threshold frequency.