Question

If a retarding potential of 1 V is applied between electrodes at what limiting wavelength $\lambda$ of light incident on the cathode will the photoelectric effect begin?

• A. $2740\stackrel{o}{A}$
• B. $2450\stackrel{o}{A}$
• C. $2540\stackrel{o}{A}$
• D. None of above

Answer 1

Answer: (C)

$2540\stackrel{o}{A}$

If retarding potential of $1V$ is applied. Then the stopping potential difference $V_o=1-0.6=0.4V$
Using $e{V}_o=E^{\prime }-\varphi$
where $E^{\prime }$ is the energy of incident light.
$\therefore$ $0.4eV=E^{\prime }-4.5eV$
$⟹E^{\prime }=4.9eV$
Wavelngth of incident light $\lambda =\frac{12413}{E^{\prime }\left(ineV\right)}{A}^o=\frac{12413}{4.9}{A}^o=2540A^o$