Which one of the following metals does not exhibit emission of electrons from its surface when irradiated by visible light?

A hydrogen atom makes a transition from $$n=5$$ to $$n=1$$ orbit. The wavelength of photon emitted is $$\lambda$$. The wavelength of photon emitted when it makes a transition from $$n=5$$ to $$n=2$$ orbit is

The curve of binding energy per nucleon as a function of atomic mass number has a sharp peak for helium nucleus. This implies that helium nucleus is

In an extrinsic semiconductor, the number density of holes is $$4 \times 10^{20} \mathrm{~m}^{-3}$$. If the number density of intrinsic carriers is $$1.2 \times 10^{15} \mathbf{m}^3$$, the number density of electrons in it is