CBSE Class 12-science Answered
if a light of certain intensity and certain frequency falls on a reflecting surface and the surface absorbs certain amount of energy say 60% then how can we calculate the force on the reflecting surface due to light ?
Asked by Dhruv | 16 Oct, 2018, 08:06: AM
Expert Answer
To get answer to this question, we need to consider n number of photons coming towards the mirror and get reflected.
In this process there is change in momentum for the reflected photons. The change in momentum is the impulse given by the mirror.
When a force F acting on a body for very small time δt, the force and the time are very difficult to measure due to the extremly short time.
what we can get is the product F×δt which is known as impulse.
This impulse is change of momentum of the body due to the action of force in very short time.
For a photon of energy hν, its momentum is hν/c (de-broglie equation ).
Initial incident intensity I = n×(hν) ..................(1)
Hence n number of photons incident on mirror and each photon carry momentum hν/c.
If 0.6×n are absorbed by mirror, then change in momentum is 0.6×n×(hν/c) .....................................(2)
impulse given by the mirror is 0.6×n×(hν/c) = 0.6×I/c ........................(3)
In eqn.(3), n is substituted using eqn.(1)
Answered by Thiyagarajan K | 16 Oct, 2018, 10:57: PM
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