(a) Calculate the momentum of a photon of light of wavelength 500 nm.
(b) Find the number of photons emitted per second by a 25 W sourse of monochromatic light of wave length 600 nm.
(c) How many photons per second does a one watt bulb emit if its efficiency is `10%` and the wavelength of light emitted is 500 nm?
(d) Monochromatic light of wavelength 300 nm is incident normally on a surface of area `4 cm^(2)` . If the intensity of light is `150 m W//m^(2)` , determine the rate at which photons strike the surface.

Explanation

(a) `p= (h)/(lambda) =(6.6xx10^(-34))/(500xx10^(-9)) =1.32xx10^(-27) kg m//sec`
(b) `p =(nhc)/(lambda)`
`n =(P lambda)/(hc) =(25xx600xx10^(-9))/(6.6xx10^(-34)xx3xx10^(8)) =7.5xx10^(19)`
(c) `Efficiency =("output power")/("input power")`
`(10)/(100) =(P)/(1) implies P=10^(-1) W`
`n =(P lambda)/(hc) =(10^(-1)xx500xx10^(-9))/(6.6xx10^(-34)xx3xx10^(8)) =2.53xx10^(17)`
(d) `n =(P lambda)/(hc) =(IA lambda)/(hc)`
`=(150xx10^(-3)xx4xx10^(-4)xx300xx10^(-9))/(6.6xx10^(-34)xx3xx10^(8))`
`=9.1xx10^(13)`