Question

How many photons does a monochromatic (means a single frequency light only) infrared range finder of power 1 mw and wavelength 1000 nm emit in 0.1s.

Answer 1

Explanation

Each photon has energyε = h

f = h c/λIf N photons are emitted,

the the total energy emitted isE = N h c /

λBecause energy equals power * time (E = P t)

we haveP t = N h c / λSoN = P t λ/ (h

c).HenceN = 10^-3 J/s

*0.1 s * 10^-6 m / (6.6 10^-34 J s * 3 10^8 m/s)~ 5.1 *10^14

So answer E is correct.