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Photons emitted by a source per second, Intensity of radiation, Photon Flux are considered the most difficult concepts.
Force exerted on a surface due to radiation are considered the most asked concepts.
39 Questions around this concept.
If a source of power 4 kW produces 1020 photons/second, the radiation belongs to a part of the spectrum called
Consider a point source of light emitting photons. And we want to find the number of Photons (n) emitted by this point source per second.
let the wavelength of light emitted by this = and
the power of the source as P (in Watt or J/s)
As we know the energy of each photon is given by
where
where c = Speed of light, h = Plank's constant =
= Frequency in Hz , = Wavelength of light.
or we can write the energy of each photon as
Then ( n=the number of photons emitted per second) is given as
The intensity of light (I) : The intensity of any quantity is defined as that quantity per unit area.
So here, light energy (or radiation ) crossing per unit area normally per second is called intensity of light energy (or radiation ).
And the intensity I is given as
Its unit is or
The intensity of light due to a point isotropic source:
Isotropic source means it emits radiation uniformly in all directions.
So The intensity I due to a point isotropic source at a distance r from it is given as
The photon flux () is defined as the number of photons incident on a normal surface per second per unit area.
As we know n ( the number of photons emitted per second) is given as
Similarly intensity I is given as
So The photon flux () is given as ratio of Intensity (I) to Energy of each photon
or
The photon flux () due to a point isotropic source at a distance r from it is given as
Radiation pressure/force- When photons fall on a surface they exert a pressure/force on the surface. The pressure/force experienced by the surface exposed to the radiation is known as Radiation pressure/force.
As we know
n=Number of emitted photons per sec is given as
where E= The energy of each photon
and Momentum of each photon is given as
And we know the force is given as rate of change of momentum.
I.e For each photon
and for n photons per sec
For a black body, we get 100 % absorption or a=1
i.e for this surface 100% of the photon will be absorbed
so
So Force is given as
where P=Power
As
So Force is given as
and radiation pressure is given as
i.e r=1
i.e for this surface 100% of the photon will be reflected
i.e
So
So Force is given as
and radiation pressure is given as
i.e body having Absorption coefficient=a and reflection coefficient=r
and we have
So Force is given as
and radiation pressure is given as
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