Photon: Overview, Questions, Preparation

The concept of photon is the fundamental principle required to understand this phenomenon of particle nature of light. It is referred to an uncharged massless particle and is the smallest unit (quantum) of electromagnetic energy. Photons can travel at the speed of light in vacuum (c=3×10^8m/s) and can carry energy and momentum, but no mass.

A photon is a quantum of electromagnetic radiation with distinct characteristics, some of which are described as follows:

1. Energy: The energy of a photon can be given by the equation:

$E=hv=\frac{hc}{\lambda }$

Where

$h=6.63\times {10}^{-34}\mathrm{J}/\mathrm{s}$

v = frequency

  $c=3\times {10}^8\mathrm{m}/\mathrm{s}$

$\lambda$  = wavelength

2. Momentum: Despite being massless, a photon has the ability to carry a momentum, which can be denoted by:

$p=\frac{h}{\lambda }=\frac{E}{c}$

3. Mass: Although a photon has zero rest mass, its relativistic mass is represented as: $m=\frac{E}{c^2}=\frac{hv}{c^2}$

This mass is non-zero when the photon is in motion.

4. Charge: Photons are electrically neutral i.e. 0 charge, unaffected by electric or magnetic fields.
   5. Speed: Photons travel at the speed of light in a vacuum, $=3\times {10}^8\mathrm{m}/\mathrm{s}$ .
5. Spin: A photons spin is given by the unit 1 (boson).

Here are a few important experiments that act as an experiment and support the theory of photons:

Photoelectric Effect:

This theory states that light waves when incident with the metal surface have the ability to eject electrons from the surface of the metal. However, the condition required for this is such that there is a certain threshold energy required for the electrons to escape the metal surface. Below this frequency, it is not possible for the electrons to be emitted regardless of the intensity of light.

Compton Scattering:

In this experiment, the electrons are scattered by the X-Rays, thereby loosing energy and changing the wavelength. Photons having high energies interact with electrons, causing them to scatter at longer distances. Hence, it clearly shows that photons have the ability to behave like particles having momentum and energy.

Blackbody Radiation:

Blackbody is an ideal object which has the ability to absorb all incoming radiation and emit it perfectly without any reflection or transmission of light. Blackbody radiation is a technique which helps to describe how objects can emit electromagnetic radiation by adjusting the temperature as required. This theory was proposed by Max Planck to overcome the limitations of classical physics. He stated that energy is quantized i.e. it can be absorbed or emitted in packets called quanta, which eventually led to the discovery of photons.

Mathematical Representation:

E=hν

Where:

• E = energy of photon
• h = Planck’s constant
• ν = frequency of radiation