Photons of wavelength 660 nm are emitted from a 60  watt lamp. What is the number of photons emitted per second?

Wavelength λ=660 nm=660×10−9 m Power P=60 watts Planks constant h=6.62×10−34 J Velocity of light C=3×108 ms-1 Number of photon per second nt=?   nt=pλhc              (∵P=nthcλ) =60×660×10−96.62×10−34×3×108 =2×1020