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} $
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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 $
