How many photons of light with a wave length of 400 pm are necessary to provide on joule of energy ?

Explanation

Wave length `lambda = 4000 p m = 4 xx 10^(-9) m`
`h = 6.625 xx 10^(-34) Js, c = 3.0 xx 10^(8) ms^(-1)`
Energy of one photon, `E = (hc)/(lambda) = (6.625 xx 10^(-34) Js xx 3 xx 10^(8) ms^(-1))/(4 xx 10^(-9)J) = 4.969 xx 10^(-17)J`
Number of photons providing 1 joule of energy
`= (1)/(4.969 xx 10^(-17)) = 2.01 xx 10^(16)`