Question

A light of wavelength $\lambda$ is incident on a metal sheet of work function $\varphi =2eV$. The wavelength $\lambda$ varies with time as $\lambda =3000+40t,where\lambda$ is in and t is in second. The power incident on metal sheet is constant at 100 W. This signal is switched on and off for time intervals of 2 minutes and 1 minute respectively. Each time the signal is switched on, the $\lambda$ start from an initial value of 3000. The metal plate is grounded and electron clouding is negligible. The efficiency of photoemission is 1%$(hc=12400eV)$. The time after which photo-emission will stop is

• A. 79 s
• B. 80 s
• C. 81 s
• D. 78 s

Answer 1

Answer: (B)

80 s

We have,
$\lambda =\frac{hc}{\varphi }$
$\therefore \lambda =6200A^o$
Also,
$\lambda =3000+40t$
$\therefore t=80s$

So, the answer is option (B).