Photon

• Photon is the particle (quantum) of electromagnetic radiation (light).
• The concept of the photon was proposed by Albert Einstein to explain the photoelectric effect.

Properties of Photon

1. Particle Nature of Light

• During interaction with matter, light behaves as if it is made up of tiny particles called photons.

2. Energy of a Photon

• Each photon has energy:

$$E = h\nu = \frac{hc}{\lambda}$$

where:

• $E$ = Energy of photon
• $h$ = Planck’s constant
• $\nu$ = Frequency
• $c$ = Speed of light
• $\lambda$ = Wavelength

3. Momentum of a Photon

• Momentum of a photon is:

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

4. Speed of a Photon

• Every photon travels with the speed of light (c = 3 × 10⁸ m s⁻¹) in vacuum.

5. Photon Energy Depends on Frequency

• Photons of the same frequency (or wavelength) have the same energy and momentum.
• Higher frequency → Higher energy
• Longer wavelength → Lower energy

6. Intensity of Light

• Increasing the intensity of light does not change the energy of a photon.
• It only increases the number of photons falling per second on a given area.

7. Electrical Nature

• Photons are electrically neutral (no charge).
• Therefore, they are not deflected by electric or magnetic fields.

8. Conservation Laws

• During photon-particle interactions (e.g., photoelectric effect or Compton scattering):
  • Total energy is conserved.
  • Total momentum is conserved.
• The number of photons may change because photons can be absorbed or emitted.

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Important Formulae

Quantity	Formula
Energy of photon	$E = h\nu = \dfrac{hc}{\lambda}$
Momentum of photon	$p = \dfrac{h\nu}{c} = \dfrac{h}{\lambda}$
Speed of photon	$c = 3 \times 10^8\ \text{m s}^{-1}$

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CBSE Important Facts

• Einstein explained the photoelectric effect using the photon theory of light.
• Light shows wave nature during propagation and particle nature during interaction with matter.
• Photon energy is independent of the intensity of light.
• Photon has energy and momentum but no rest mass and no electric charge.