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" pm"=4xx10^(-9)m`
`h=6.625xx10^(-34)Js, c=3.0xx10^(8)ms^(-1)`
`"Energy of one photon, E "=(hc)/(lambda) =`
`(6.625xx10^(-34)Jsxx3xx10^(8)ms^(-1))/(4xx10^(-9)J)=4.969xx10^(-17)J`
 Number of photons providing 1 joule of energy
`=(1)/(4.969xx10^(-17))=2.01xx10^(16)`