Question

When a centimeter thick surface is illuminated with light of wavelength $\lambda$, the stopping potential is V. When the same surface is illuminated by light of wavelength $2\lambda$, the stopping potential is $\frac{V}{3}$. The threshold wavelength for the surface is

• A. $\frac{4\lambda }{3}$
• B. $4\lambda$
• C. $6\lambda$
• D. $\frac{8\lambda }{3}$

Answer 1

The correct option is B $4\lambda$

As per the question, $eV=\frac{hc}{\lambda }-\varphi$
and $\frac{eV}{3}=\frac{hc}{2\lambda }-\varphi$
Solving, we get $\varphi =\frac{hc}{4\lambda }$
Hence, threshold wavelength is $4\lambda$.