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

Explanation

wavelength `lambda=4000 pm = 4xx 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 1a0^(-17) J`
Number of photons providing 1 joule of energy
`= (1)/(4.969 xx 10^(-17) ) = 2.01 xx 10^(16)`