If 2.5×10-9 N average force is exerted by a light wave on a non- reflecting surface of 30 cm2 area during 40 minutes of time span, the energy flux of light just before it falls on the surface is ................ W / cm2. (Round off to the Nearest Integer) (Assume complete absorption and normal incidence conditions are there)
If 2.5×10-9 N average force is exerted by a light wave on a non- reflecting surface of 30 cm2 area during 40 minutes of time span, the energy flux of light just before it falls on the surface is ................ W / cm2. (Round off to the Nearest Integer) (Assume complete absorption and normal incidence conditions are there)
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1 Answer
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The force (F) exerted by radiation is the rate of change of momentum (p).
F = Δp/Δt
For photons, p = E/c. So, F = (1/c) * (ΔE/Δt).
Since Power (P) is ΔE/Δt, F = P/c.
Intensity (I) is Power per unit Area (P/A).
The formula provided in the document is F/A = (nE)/ (Δt c A) which leads to a final calculated value of 25 W/cm².
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