The Photoelectric effect
Another unexplained phenomenon in 1900
When light is shined on metallic objects, electrons are ejected from their surface
- the electrons come off with a range of energies
up to a maximum energy, which depends on the
- the intensity of the light does not determine the
maximum energy of the out-coming electrons, it
determines only their number
- experimental results in contradiction with the wave-model of light
Conflict solved by A. Einstein, using Planck’s hypothesis
- light carries energy in bundles, called photons
- the energy of a photon for an electromagnetic wave with frequency f is: E=h f
- a photon can give either no energy or gives its entire energy to an atom, which can transfer this energy to an electron
New conflict: light is BOTH particle and wave
Old conflict: still no good model for the light