The emission of free electrons from a metal surface when the light is shone on it, it is called the photoemission or the photoelectric effect. This effect led to the conclusion that light is made up of packets or quantum of energy. Now the question was whether the light quantum theory was indicative of the particle nature of light. Einstein already associated the light quantum with momentum. This strongly supported the particle nature of light and these particles were named photons. Thus, the wave-particle duality of light came into the picture.
Photoelectric Emission
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Photons:
Some points to be kept in mind are:
- A photon is an elementary particle. It is a quantum of light.
- Energy of a photon is given by \(\begin{array}{l}E = h\nu\end{array} \). Its momentum is\(\begin{array}{l}p = \frac{h\nu }{c}\end{array} \)and speed is c, which is the speed of light.
- Irrespective of the intensity of radiation, every photon of a frequency v has the same momentum \(\begin{array}{l}p = \frac{h\nu }{c}\end{array} \)and energy\(\begin{array}{l}E = h\nu\end{array} \).
- The increase in the intensity of light only increases the number of photons crossing an area per unit time. It does not affect the energy of the radiation.
- A photon remains unaffected by electric and magnetic fields. It is electrically neutral.
- A photon has a zero mass, i.e. it is massless.
- It is a stable particle.
- Photons can be created or destroyed when radiation is emitted or absorbed.
- The total energy and momentum are conserved during a photon-electron collision.
- A photon cannot decay on its own.
- The energy of a photon can be transferred during an interaction with other particles.
- A photon is a spin-1 particle, unlike electrons which are ½ spin. It’s spin axis is parallel to the direction of travel. It is this property of photons which supports the polarization of light.
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