Question

A parallel beam of uniform, monochromatic light of wavelength 2640 $\stackrel{o}{A}$ has an intensity of 100 $W/m^2$. The number of photons in 1 $m{m}^3$ of this radiation are -

• A. 266
• B. 335
• C. 442
• D. 555

Answer 1

The correct option is A 266

Wavelength $=2640A$

Intensity $=$ Power / Area $=200$

Area $=1{mm}^2={10}^{-6}{m}^2$

Energy of ${10}^{-6}{m}^2$ area $=200\times {10}^{-6}=2\times {10}^{-4}J/s$

No. of photons $=2\times {10}^{-4}/7.5\times {10}^{-19}=2.66\times {10}^{14}$