Equation #14: Photoelectric Effect
Albert Einstein was awarded the Nobel prize for his explanation of the photoelectric effect: when light is shone on a metal surface, electrons are ejected. However, the light cannot be of just any wavelength, it has to be lower (and frequency higher) than the threshold wavelength.
In practical terms, it means that no matter how long you shine light of lower frequency (e.g. visible or infrared light for Zinc surface) on the metal surface, no change happens even if that light is very bright. However, even a faint radiation of higher frequency (e.g. UV light for Zinc surface) can cause observable effect when shone even for a few seconds.
The explanation is given in terms of photon picture of light, We assume that light is made up of photons which all have a particular frequency (and thus energy). When a photon interacts with an electron at the metal surface, it imparts this energy to the electron. However, electrons require some minimum energy to be able to escape out of the metal, and it can absorb only one photon at a time. Consequently, you cannot make it escape by providing photons of lower energy, no matter how many.
A simple demonstration:
Explanation by analogy with Elephants:
For more equations like this, see Famous equations in Physics