^Momentum of photons
^Momentum of photons
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^Energy of photons
^Energy of photons
Max plank suggested that any source of radiation emits or absorbs energy discontinuously in the form of energy particles or packets of energy called photons. Amount of energy of an EM wave of frequency f possessed by a single photon is,
Total energy of EM wave having N photons is
E = Nhf.
Here h is Planck’s constant. It’s value is
h = 6.63 x 10–34 Js = 4.14 x 10–15 eVs
Dimensions of Planck’s constant & angular momentum are same & are [ML2 T – 1].
^Frequency of photons
^Frequency of photons
Frequency of photon does not change when it travels in different media while there is a change in velocity of photon in different media (due to change in wavelength). For any wave frequency is related to its wave length as 
i.e. light of shorter wavelength has more frequency.
^What are photons
^What are photons
Photons are electrically neutral energy particles of zero rest mass emitted by a source of radiation & travel along a straight line with a speed equal to speed of light. Zero rest mass implies that the photons can’t exist at rest. Photons travel in a straight line with a speed equal to speed of light. Being electrically neutral, photons are not deflected by electric and magnetic fields.
^Photons & neutrino
^Photons & neutrino
Both photon & neutrino are charge less & zero rest mass. But the spin of photon is 1 & that of neutrino is 1/2.
^Appearance of sky
^Appearance of sky
The appearance of sky from earth at various locations depend on the amount & nature of impurities present in the air & climatic conditions also.
Also it is found that the light scattered
(a) at right angles to the direction of incidence is completely plane polarized light (CPL)
(b) along the direction of incidence is unpolarized, &
(c) at all other angles to the direction of incidence is partially plane polarized light (PPL)
On the Moon, there is no blue sky because there is no atmosphere & thus no scattering. Even during the day, the sky is as black as at night, although the Sun and the Earth may be brightly shining.
If we look at the blue portion of the sky through a Polaroid and rotate the polariod, the transmitted light shows rise and fall of intensity, which indicates that the light from the blue portion of the sky is plane polarized.
^Clouds appears white
^Clouds appears white
Hence when white light encounters obstacles of size a >> λ, all colours are scattered equally & resultant scatted light appears white. Due to this reason clouds appears white.
If the earth had no atmosphere, the sky would appear black and stars could be seen during day time also. In fact at a height of above 15 km, there is almost no air & the appears black.
^Sun appears red at Sun rise & sun set
^Sun appears red at Sun rise & sun set
This is because at Sun rise & sun set light passes through greater thickness of atmosphere, as a result more scattering takes place & blue colour & low wavelengths are scattered away by the scattering caused by the atmosphere & the colour which reaches us is unscattered red.
General scattering
When size of obstacles encountered by the light is much greater than the wavelength of light used, all wavelengths of light are scattered equally.
^Dangers signals are made red
^Dangers signals are made red
Danger signals, traffic lights etc. are made red. Red light being longest wavelength is scattered least & hence can go to large distance without any appreciable loss due to scattering.
^Scattering of light
^Scattering of light
When unpolarized light falls on a gas or air molecules light falling on air molecules. The electric field of light polarises the air molecule makes it an electric dipole, which starts oscillating under the influence of oscillatory electric field of light & these dipoles radiate electromagnetic waves in all directions. This process is called scattering of light & the radiated light is called scattered light, it is unpolarized & intensity is found to be strongest along a line perpendicular to the oscillation, & drops to zero along the line of oscillation. The amount of scattering depends on the obstacle size coming in its way e.g. dust particles, rain drops, ice particles etc. We have divided scattering in two type.
1. Rayleigh scattering
When size of obstacles encountered by the light is much smaller than the wavelength (a << λ) of light used, the amount of scattering is inversally proportional to the fourth power of the wavelength of light.
