Question

How many photons are emitted during 6.0 s of operation of a red laser pointer? The device outputs 2.0 mW at a 635 nm wavelength. Choose best answer.

(a) 3.8×10¹⁰
(b) 3.8×10¹¹
(c) 3.8×10¹⁵
(d) 3.8×10¹⁶

Answer 1

Data:

Time, t = 6 s

Power, P = 2 x 10^-3 W

Wavelength, λ = 635 x 10^-9 m

Let

n = No. of photons

Solution:

Energy of a photon, E = h c / λ

                                  = 6.6 x 10^-34 * 3 x 10^8 / 635 x 10^-9

                                  = 3.12 x 10^-19 J

Power, P = n E / t

No. of photons, n = P t / E

                            = 2 x 10^-3 * 6 / 3.12 x 10^-19

                            = 3.8 x 10^16

Ans:

No. of photons, n = 3.8 x 10^16