Question

The number of photons per second on an average emitted by the source of monochromatic light of wavelength 600 nm, when it delivers the power of 3.3 × 10^–3 watt will be : (h = 6.6 × 10^–34 Js)

(1) 10¹⁸

(2) 10¹⁷

(3) 10¹⁶

(4) 10¹⁵

Answer 1

Correct option is (3) 10¹⁶

The power of a source is given as,

\(P = \frac Et \)

\(= \frac nt (\frac {hc} \lambda)\)

⇒ \(\frac nt = \frac P{\left(\frac {hc}{\lambda}\right)}\)

(Here \(\frac nt\) is number of photons emitted per second)

⇒ \(\frac nt = \frac{3.3 \times 10^{-3} \times 600 \times 10^{-9}}{6.6 \times 10^{-34} \times 3 \times 10^8}\)

\(= 10^{16}\) photons/sec.

Answer 2

Option : (3) 10¹⁶

p = \(\frac{nhc}{λ}\) 

⇒ n = \(\frac{pλ}{hc}\)