2.0 Characteristics of photon

  Basic Modern Physics

• A photon always travels at a speed of $c = 3 \times {10^8}m/s$ in vacuum and it is independent of any form of reference frame used to observe the photon.

• A definite energy and momentum is associated with each photon. $$E = hf = \frac{{hc}}{\lambda }$$

• Rest mass of photon is zero (as it never exists at rest). Equivalent mass of photon: $$\begin{equation} \begin{aligned} E = m{c^2} \\ E = hf \\\end{aligned} \end{equation} $$ So, $$m{c^2} = hf$$ or $$\begin{equation} \begin{aligned} m{c^2} = \frac{{hc}}{\lambda } \\ m = \frac{h}{{c\lambda }} \\\end{aligned} \end{equation} $$

• Photons have momentum: $$\begin{equation} \begin{aligned} p = mc \\ p = \frac{h}{{c\lambda }}c = \frac{h}{\lambda } \\\end{aligned} \end{equation} $$

• With change in medium, the speed and wavelength of the photon changes but frequency remains the same.

• When a photon collides with a particle, energy and momentum is conserved while number of photons may not be conserved (it can be absorbed or a new photon may be emitted).

• On increasing intensity photon count per unit area increases while the energy of each photon remains the same (because on increasing or decreasing intensity, frequency doesn't changes)

• They are electrically neutral. They are not deflected by electric and magnetic field.

• $h = 6.626 \times {10^{ - 34}}JS$ known as plank's constant
• The above given formula is only applicable to photons only. Never use it to find the energy of particle.