Question

Work function of lithium and copper are respectively $2.3$ and $4.0eV$. Which one of the metals will be useful for the photoelectric cell working with visible light ? $(h=6.6\times {10}^{-34}J-s,c=3\times {10}^{8}m/s)$

• A. Lithium
• B. Copper
• C. Both
• D. None

Answer 1

Answer: (A)

The approximate range of visible spectrum is $400$ to $700nm$.
The minimum energy required for the emission of photoelectrons from a metal is called the work function of that metal.
$W=\frac{hc}{\lambda }$
$\Rightarrow$ $\lambda =\frac{hc}{W}$
$=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{2.3\times 1.6\times {10}^{-19}}$
$=5.38\times {10}^{-7}m=538nm$ (for lithium)
For copper $\lambda =\frac{hc}{W}$
$=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{4\times 1.6\times {10}^{-19}}=309nm$
Since, wavelength range of visible spectrum is $400nm$ to $700nm$,
have only lithium lies in this range. Hence, lithium is useful.