In the early 20th century, the luminous phenomena exhibited some disturbing facts, including, for example:
By irradiating the photocathode covered with an alkali metal, one managed in some cases to extract electrons wich jumped on the anode and thereby generated an electric current detected by the galvanometer G. It was found that this "photoelectric effect" occurs only from a certain frequency of the incident light on.
How to explain the photoelectric effect?
As no scientist came to explain the photoelectric effect with the wave theory of light, Albert Einstein used a hypothesis of Max Planck to give it a simple but revolutionary explanation:
Light is composed of particles (called photons). Photons have a frequency proportional to the energy of light: Photon energy: $E$ $ =$ $ h\cdot\nu \;(7)$ Planck's constant $h$ is a proportionality constant. $h$ $=$ $6.626\cdot10^{- 34}J\cdot s \;(8)$
The light of a given color is more or less intense according to the number of photons transported.
The photoelectric effect is so easily explained:
No "red" photon has enough energy to remove an electron from the alkali metal of the photocathode (low frequency). Although red light is very intense (many red photons) no electrons will be ripped off. (See 1000 people trying to pull a nail from a wooden board with their fingers ...) Each purple photon has enough energy to remove an electron. (high frequency). Although violet light is less intense (few purple photons ) electrons are torn out, resulting in a current. (Assume that a single person shows up with pliers to pull the nail ...)
Light is a corpuscular phenomenon Light is both a corpuscular and wave phenomenon.
Photons are particles-waves which escape the intuitive picture that we have of the world. These beings obey only (and hopefully anyway) the laws of mathematics !!