Q. In photoelectric effect, the threshold wavelength of sodium is $5000\ \mathring{\mathrm{A}}$. Find its work function$(h = 6.6 \times 10^{- 34}\ Js,\ c = 3 \times 10^{8}\ {ms}^{- 1},\ 1\ eV = 1.6 \times 10^{- 19}\ J)$
A) 7.5 eV
B) 2.5 eV
C) 10 eV
D) 5.0 eV
Explanation
(b) The work function of sodium
$W = \frac{hc}{\lambda}$
$W = \frac{6.6 \times 10^{- 34} \times 3 \times 10^{8}}{5 \times 10^{- 7}}$
or $\ W = 3.96 \times 10^{- 19}$J
or $W = 2.47eV\ \ $ ($\because 1eV = 1.6 \times 10^{- 19}$ J)
or $W = 2.5eV(approximately)$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.