PARTICLE NATURE OF LIGHT: THE PHOTON -

In 1905 Einstein proposed a new theory of light called photon theory of light.

According to this, photons are the packets of energy which are emitted by a source of radiation and when they interact with matter, it behaves like a particle.

The energy of each photon is given by $\fn_jvn \left [ E=h\nu \right ]$

where $\fn_jvn h\rightarrow$ Plank constant ( $\fn_jvn 6.62\times10^{-34}Js$

$\fn_jvn \nu\rightarrow$ Frequency of radiation

*Features of the photon theory of light:-

1. Each photon has energy ( $\fn_jvn E=h\nu$ ) and momentum ( $\fn_jvn p=h\nu$ ) and speed ( $\fn_jvn c=3\times10^8 m/s$ ) in free space.

2. When the intensity of the light beam is increased, the number of photons in the beam increases, but the energy and momentum of each photon are not changed.

3. Photons exist only while moving and have zero rest mass.

The mass of a particle moving with velocity V is given by.

$\fn_jvn m=\frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}$

where $\fn_jvn m_0$ -> rest mass

Note that, a photon is never at rest.

4. The only mass that a photon has it due to its Kinetic energy

Now $\fn_jvn E=mc^2$

$\fn_jvn \Rightarrow h\nu=mc^2$

$\fn_jvn \therefore \left [ m=\frac{h\nu}{c} \right ]$

5. The linear momentum of a photon is given by

$\fn_jvn p=mc$

$\fn_jvn =\left ( \frac{h\nu}{c^2} \right )\times c$

$\fn_jvn \left [ p=\frac{h\nu}{c} \right ]$

$\fn_jvn \left [ p=\frac{h}{\lambda} \right ]$

6. Photons are electrically neutral and are not deflected by electric and magnetic fields.

7. The velocity of a photon is different in different mediums.

8. In a photon – particle collision, the total energy, and momentum remain conserved. However, the number of photons may not be conserved in a collision. The photon may be absorbed or a new photon may be created.

QUESTION:> WHAT IS PHOTOCELL? DESCRIBE IT.

ANS:> A photocell some times called electric eye is a device that converts light energy into electric energy. It is based on the photoelectric effect.

When the light of frequency greater than the threshold frequency of cathode material is incident on the cathode C, electrons are emitted from it.

These electrons are attracted by the +ve anode to constitute current in the external circuit. The magnitude of the current is of the order of $\fn_jvn \left ( \mu A \right )$ . The magnitude of the current can be increased by using the DC amplifier.

APPLICATION:-

a. Photocells are used for switching On or OFF the street light automatically.

b. Photocells are used in burglar alarm and fire alarm.

c. Because of their high sensitivity, Photocells find wide applications in scientific research ex- to study the intensity of the light source, to find the illuminating power, etc.

d. Photocells are used in television cameras.

e. Photocells are used in counting operation.

f. Photocells are used in cameras for the automatic adjustment of the aperture.

g. Photocells are used to determine Planck’s constant.

IMPORTANT LINKS OF DUAL NATURE OF RADIATION AND MATTER
Introduction	Electron Emission
Photoelectric Effect	Experimental Study of Photoelectric Effect
Photoelectric Effect and Wave Theory of Light	Einstein’s Photoelectric Equation: Energy Quantum of Radiation
Particle Nature of Light: The Photon	Wave Nature of Matter
Davisson and Germer Experiment