What is a photon ( Particle Nature of Light )? Write the properties of a photon.
When radiation interacts with matter, to cause emission of electrons, the radiation behaves as if it is made up of particles called photons. Thus, dual nature of radiation (i.e., wave and particle nature) was established.
Photon is a packet of energy or quantum of energy ejected at the speed of light by an emitter.
The energy of each bundle or packet or a photon is given by,
$E = h\nu$
where $h$ is Planck's constant
$h= 6.63 \times 10^{-34} Js$ and $\nu$ is the frequency of radiation.
Energy of $n$ photons is given by
$E = nh\nu$
A source of radiation emits energy in packets when it goes from higher energy state to lower energy state and absorbs energy in packets when it goes from lower energy state to higher energy state.
Properties of a Photon :
(i) A photon travels with a speed of light in vacuum (i.e. $3 \times 10^8 ms^{-1}$).}
(ii) Rest mass of a photon is zero i.e. $m_0 = 0$. It means that photon cannot exist at rest. Mass of a particle ($m$) moving with velocity $v$ is given by
$m = \frac{m_0}{\sqrt{1 - (v^2/c^2)}}$
(where $m_0$ is the rest mass of the particle and $c$ is the velocity of light.)
Since, a photon moves with velocity of light in vacuum i.e. $v = c$, so $m_0 = 0$ from the above relation.
(iii) The kinetic mass of a photon is
$m = \frac{E}{c^2} = \frac{h\nu}{c^2} = \frac{h}{c\lambda}$.
(iv) The magnitude of momentum of a photon is, $P = mc = \frac{E}{c} = \frac{h\nu}{c} = \frac{h}{\lambda}$.
The direction of the momentum of a photon is same as that of the direction of the radiation.
(v) Energy of photon in terms of wavelength can be worked out as below:
If $\lambda$ is the wavelength of the radiation, then $c = \nu\lambda$ or $\nu = c/\lambda$.
Hence, energy of photon is given by:
$E = h\nu = \frac{hc}{\lambda}$
i.e.
$E = \frac{6.62 \times 10^{-34} \times 3 \times 10^8}{\lambda} J = \frac{19.8 \times 10^{-26}{ \lambda (m)}$
$E= \frac{1242 eV}{\lambda (\text{in } nm)}$
Energy of a photon is usually measured in eV (i.e., electron volt).
(vi) Photons travel in straight lines.
(vii) Wavelength of the photon changes in different media so velocity of a photon is different in different media.
(viii) Photons do not have any charge so they are electrically neutra
(ix) Photons are not deflected by magnetic fields.
(x) Photons are not deflected by electric fields.
(xi) Photons may show diffraction under given conditions.
(xii) Total energy and total momentum of photons remain conserved when photons collide with matter. All photons having same frequency have same energy and momentum irrespective of the intensity of the radiation.