What is the number of photons of light with a wave length 4000 pm that provide 1J energy ?

Explanation

wavelength, `lambda = 4000 p m = 4 xx 10^(-19)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))/(4xx 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)`