### Neet Solved Paper 2015 Question 40

##### Question: A certain metallic surface is illuminated with monochromatic light of wavelength $ \lambda $ . The stopping potential for photoelectric current for this light is $ 3V _0 $ . If the same surface is illuminated with light of wavelength 2 $ \lambda $ , the stopping potential is $ V _0 $ . The threshold wavelength for this surface for photoelectric effect is

#### Options:

A) $ 6\lambda $

B) $ 4\lambda $

C) $ \frac{\lambda }{4} $

D) $ \frac{\lambda }{6} $

## Show Answer

#### Answer:

Correct Answer: B

#### Solution:

From photoelectric equation $ hv+W+eV _0 $ (where, W = work function)

So $ \frac{hc}{\lambda }=,W+3eV _0 $ …(i)

Also, $ \frac{hc}{2\lambda }=W+eV _0 $

$ \Rightarrow \frac{hc}{\lambda }=2W+2eV _0 $ Subtracting Eq. (i) from Eq. (ii), we get $ 0=W-eV _0 $

$ \Rightarrow W=eV _0 $ From Eq. (i), $ \frac{hc}{\lambda }=eV _0+3eV _0=4eV _0 $

The threshold wavelength is given by $ {\lambda _{th}}=\frac{hc}{W}=\frac{4eV _0\lambda }{eV _0}=4\lambda $