JEE MAIN - Physics (2004 - No. 3)
The work function of a substance is $$4.0$$ $$eV.$$ The longest wavelength of light that can cause photo-electron emission from this substance is approximately.
$$310$$ $$nm$$
$$400$$ $$nm$$
$$540$$ $$nm$$
$$220$$ $$nm$$
Explication
For the longest wavelength to emit photo electron
$${{hc} \over \lambda } = \phi \Rightarrow \lambda = {{hc} \over \phi }$$
$$ \Rightarrow \lambda = {{6.63 \times {{10}^{ - 34}} \times 3 \times {{10}^8}} \over {40 \times 1.6 \times {{10}^{ - 16}}}} = 310nm$$
$${{hc} \over \lambda } = \phi \Rightarrow \lambda = {{hc} \over \phi }$$
$$ \Rightarrow \lambda = {{6.63 \times {{10}^{ - 34}} \times 3 \times {{10}^8}} \over {40 \times 1.6 \times {{10}^{ - 16}}}} = 310nm$$