A photoemissive substance is illuminated with a radiation of wavelength $\lambda_i$ so that it releases electrons with de-Broglie wavelength $\lambda_e$. The longest wavelength of radiation that can emit photoelectron is $\lambda_0$. Expression for de-Broglie wavelength is (m = mass of electron, h = Planck's constant, C = Speed of light)

An electron makes a transition from an excited state to the ground state of a hydrogen like atom. Out of the following statements which one is correct ?
(A) K.E, P. E. and T.E. decreases
(B) K.E. increases but P. E. and T.E. decreases
(C) K.E. and T.E. decreases but P.E. increases
(D) K.E. decreases, P.E. increases but T.E. remains the same

The ratio of the density of oxygen nucleus $({}^{16}_{8}\text{O})$ and helium nucleus $({}^{4}_{2}\text{He})$ is

Electrical conductivity of insulators is