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Photons emitted by a source per second, Intensity of radiation, Photon Flux are considered the most difficult concepts.
69 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
A point source of light emits photons of energy 3.5 eV each. If the power of the source is 2.0 W, then the number of photons emitted per second is:
A light bulb emits photons of various wavelengths ranging from 400 nm to 700 nm. If the power of the light bulb is 100 W, what is the total number of photons emitted per second?
A 100 W light bulb emits photons per second. Determine emitted photons' wavelength.
and
A laser diode emits light of wavelength with a power of P = 20 mW. How many photons are emitted per second from the laser diode?
Photons absorbed in the matter are converted to heat. A source emitting n photon/sec of frequency ν is used to convert 1kg of ice at 0°C to water at 0°C. Then, the time T taken for the conversion
(a) decreases with increasing n, with ν fixed
(b) decreases with n fixed, ν increasing
(c) remains constant with n and ν changing such that nν = constant
(d) increases when the product nν increases
In a photoelectric effect experiment, the graph stopping potential V versus the reciprocal of wavelength obtained is shown in the figure. As the intensity of incident radiation is increased:
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The intensity of radiation from the source depends upon the distance 'd' of the point of observation from the source as
A photocell is illuminated by a small bright source placed 1 m away. When the same source of light is placed (1/2) m away, the number of electrons emitted by photocathode would
A radiation of energy falls normally on a perfectly reflecting surface. The momentum transferred to the surface is :
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
the power of the source as P (in Watt or
As we know the energy of each photon is given by
where
where
or we can write the energy of each photon as
Then (
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
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
As we know
Similarly intensity I is given as
So The photon flux
- The photon flux
The photon flux
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
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
i.e for this surface
So Force is given as
where
So Force is given as
and radiation pressure is given as
i.e For black body,
- For perfectly reflecting surface (i.e mirror)
i.e
i.e for this surface
i.e
So Force is given as
and radiation pressure is given as
- For neither perfectly reflecting nor perfectly absorbing body
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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