Photoelectrons are liberated by ultraviolet light of wavelength $3000A^{\large\circ}$ from a metallic surface for which the photoelectric threshold is 4000$A^{\large\circ}$.Calculate De-Broglie wavelength of electrons emitted with maximum kinetic energy.

Question

$\begin{array}{1 1}(a)\;1.2\times 10^{-7}m&(b)\;1.2\times 10^{-9}m\\(c)\;2.3\times 10^{-9}m&(d)\;1\times 10^{-3}m\end{array}$