How many photons of light with a wave length of 4000 pm are necessary to provide one joule of energy? 

Explanation

Wave length `lambda = 4000 p m = 4 xx 10^(-19) m, h= 6.626xx10^(-34) Js, c = 3.0xx10^(8) ms^(-1)`
Energy of one photon,`E=(hc)/(lambda)=(6.626xx10^(-34)Jsxx3xx10^(8)ms^(-1))/(4.969xx10^(-17)J)`
 Number of photons providing 1 joule of energy  `=(1)/(4.969xx10^(-17))=2.01xx10^(16)`