Question

Light of wavelength l strikes a photo-sensitive surface and electrons are ejected with kinetic energy E. If the kinetic energy is to be increased to 2E, the wavelength must be changed to l' where –

• A. l' = $\frac{\lambda}{2}$
• B. l' = 2l
• C. $\frac{\lambda}{2}$<l' <l
• D. l' >l

Answer 1

Answer: (C)

$\frac{\lambda}{2}$<l' <l

Answer 2

If l¢ = l/2 ̃ E₂ > 2E₁

But here E₂ = 2E₁ So l¢ > l/2

So answer is $\lambda > \lambda ^ { \prime } > \lambda / 2$