What is the energy of an electron in the stationary state corresponding to $n=2$?

What is the radius of the stationary orbits $(n)$? State the formula.

If the shortest wavelength of the hydrogen atom in the Lyman series is $x$,then the longest wavelength in the Balmer series of $He^+$ is:

The graph that depicts Einstein's photoelectric effect for a monochromatic source of frequency above the threshold frequency is

Explain the phenomenon of black body radiation.

In astronomical observations,signals observed from the distant stars are generally weak. If the photon detector receives a total of $3.15 \times 10^{-18} \, J$ from the radiations of $600 \, nm$,calculate the number of photons received by the detector.