Photoelectric Effect

Let the Work Function of the surface of the photo-emissive cell be $W$ .

By Einstein's equation of Photo-electricity, we have:

$\frac{hc}{\lambda} = W + K$ , where $K$ is the max. K.E when light of wavelength $\lambda$ is incident on it

$\Rightarrow \frac{hc}{\lambda}> K$ , since $W >0$

And, when light of wavelength $\frac{3\lambda}{4}$ is incident on the same surface, we have:

$\frac{4hc}{3\lambda} = W + K'$ , where $K'$ is the max. K.E when light of wavelength $\frac{3\lambda}{4}$ is incident on it

Subtracting the two equations, we get: $K' = \frac{hc}{3\lambda} + K > \frac{K}{3} + K = \frac{4K}{3}$