Question

Write the 4 laws of photoelectric effect

Answer 1

The four laws of photoelectric effect mention about the Intensity dependence of photocurrent, Its frequency dependence, Energy relation and the instantaneous nature of photoelectric emission. Recall the observations of photoelectric effect experiments.

Complete step by step answer:
It was observed that If we shine a light on a metal surface, electrons were ejected from the surface and this phenomenon was called the photoelectric effect. The wave theory of light could not explain the phenomenon completely and a new photon model of light was introduced. The laws of photoelectric effect were made by Albert Einstein and they are :

1. Electromagnetic radiations transfer their energy in discrete units, each of energy $h\nu$. To remove an electron from the metal, some minimum amount of energy called the work function ${W_0}$ has to be supplied to the metal. If the energy of incident photons are greater ${W_0}$, electrons are ejected. The extra energy goes to the electrons as their kinetic energy.
$h\nu = {W_0} + KE$

2. There exists a characteristic threshold frequency, corresponding to the work function of the metal. Light below this frequency cannot transfer the required energy and hence cannot exhibit photoelectric effect.

3. The intensity of a light beam is decided by the number of photons falling over a given area of the metal. Since more photons can interact with more electrons, The photo-current is proportional to the Intensity of light falling given that the frequency of the radiation is above threshold frequency.

4. The photoelectric emission is almost instantaneous since each photon transfers its energy completely to a single electron.

Note: The wave theory of light could not explain the phenomenon because in a wave picture, the energy of a wave is dependent on intensity. But here, emission occurred only above a certain frequency no matter how high the intensity is. It could also not explain why the emissions were instantaneous as it would take a significant time for the small portion of wave front that interacts with an electron to transfer the required energy.