^Power of photons

Rate of energy of photons is called power of photons. If n photons each of wavelength ‘l’ are crossing a given area at right angles every second, then power of photons is defined as,

^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}].

^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.

^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.