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A monochromatic beam of light (lamda=490...

A monochromatic beam of light `(lamda=4900A)` incident normally upon a surface produces a pressure of `5xx10^(-7)Nm^(-2)` on it. Assume that `25%` of the light incident in reflected and the rest absorbed. Find the number of photons falling per second on a unit area of thin surface

Text Solution

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`P=[2(0.25)+0.75](I)/(c )=1.25(I)/(c )`
`:. ` Intensity of light, `I=(cP)/(1.25)=((3xx10^(8))(5xx10^(-7)))/(1.25)=120Wm^(-2)`
Energy of photon, `E=(hc)/(lambda)=((6.63xx10^(-34))(3xx10^(8)))/(0.49xx10^(-6))=4xx10^(-19)J`
`:. ` Number of photons incident per unit area per second
`n=(I)/(E)=(120)/(4xx10^(-19))=3xx10^(20)m^(-2)s^(-1)`
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