Is [[Light|light]] [[Wave-Particle Duality|a wave or a particle]]? This was one of the main questions in physics at the beginning of the 20th century. Experiments like the [[Double Slit Experiment|double slit experiment]] suggested that light is a [[Wave|wave]]: it [[Interference|interferes]] and shows [[Diffraction|diffraction]] when passing a small slit. However, not all experiments agreed with that explanation. One of them is the *photo-electric effect*. The experimental setup for the *photo-electric effect* is rather simple; it consists of only two ingredients: a light source and a sheet of metal. ![[photo_electric_effect.excalidraw.light.svg]] If one shines light onto the metal plate, we see a different result depending on the [[Frequency|frequency]] for the light. Light at low frequency (red light), nothing happens. However, if we shine blue light (high frequency, right picture) on the same plate, electrons are released. If we assume that light is a wave, all frequencies of light should release electrons, at low frequency we might just need more intensity, i.e. brighter light. More graphically: even if you shine a stadium floodlight on the metal plate, you will not see electrons leaving, if the frequency of the light is too low. This observation is called the *photoelectric effect*. The discovery of the photoelectric effect was one of the first experimental clues for [[quantum mechanics]]. The best explanation is that light consists of small particles, called [[Photon|photons]]. Their energy depends on the frequency of the light. Only if a single [[Photon|photon]] has enough energy to overcome the binding energy of an [[Electron|electron]], electrons are released. It was for the explanation of this effect that Einstein got he 1921 Nobel prize (not for the theory of relativity). >[!read]- Further Reading >- [[Quantum Mechanics]] >- [[Discreteness]] >- [[Black Body Radiation]] >[!ref]- References >- A. Einstein, Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt, Ann. Phys. **322**, 132 (1905).