question_answer

A beam of light has two wavelengths of 4972Å and 6216Å with a total intensity of \[3.6\times {{10}^{-3}}W{{m}^{-2}}\] equally distributed among the two wavelengths. The beam falls normally on an area of 1 \[c{{m}^{2}}\] of a clean metallic surface of work function 2.3 eV. Assume that there is no loss of light by reflection and that each capable photon ejects one electron. The number of photoelectrons liberated in 2s is approximately:          [JEE ONLINE 12-04-2014]

A) \[6\times {{10}^{11}}\]

B) \[9\times {{10}^{11}}\]

C) \[11\times {{10}^{11}}\]

D) \[15\times {{10}^{11}}\]

Correct Answer: B

Solution :

[b] Given, and

and

Intensity associated with each wavelength

work function

for different wavelengths

Work function for metallic surface  (given)

Therefore, will not contribute in this process.

Now, no. of electrons per  no. of photons per

no. of electrons per

So, the number of photo electrons liberated in 2 sec.