Question

Work function of a metal is 2.1 eV. The pair of wavelengths which is able to emit photo-electrons is

• A. 4000 $A^0$, 7500 $A^0$
• B. 5500 $A^0$, 6000 $A^0$
• C. 4000 $A^0$, 5000 $A^0$
• D. None of these

Answer 1

Answer: (C)

4000 $A^0$, 5000 $A^0$

The maximum wavelength required for metals to eject photons ,
$\frac{hC}{\lambda }=2.1eV$
$\Rightarrow \frac{4.41\times {10}^{-15}\times 3\times {10}^8}{\lambda }=2.1$
$\Rightarrow \lambda =6.3\times {10}^{-7}m\Rightarrow \lambda =6300A^{\circ }$
So,
Both The wavelengths should be less that $6300A^{\circ }$.
So, the answer is option (C).