Question

A laser used to read CDs emits red light of wavelength 700 nm. How many photons does it emit each second if its power is?(a) 0.10 W, (b) 1.0 W

Answer 1

power = work/time

watt = joule/sec

a) power = 0.10 watt = 0.1joules/sec

work = power * time

= 0.10 * 1 =0.1joules

work is northing but energy

therefore energy is 0.1joules

according to planks quantum theory E = nhν where nis the no. of photons ; h is planks constant; ν isfrequency

hence, n = E/hν

or n = Eλ/hc (ν = c/λ)

n = (0.1*700*10^-9)/6.625*10^-34*3*10^8 = 0.3522*10^18photons

b) similar to the above calculation

here E = 1.0joules

n = (1.0*700*10^-9)/6.625*10^-34*3*10^8 = 0.3522*10^17photons