What is a black body?

Electromagnetic radiation with maximum wavelength is

When electromagnetic radiation of wavelength $300 \, nm$ falls on the surface of a metal, electrons are emitted with a kinetic energy of $1.68 \times 10^5 \, J \, mol^{-1}$. What is the minimum energy (work function) needed to remove one mole of electrons from the metal? $(h = 6.626 \times 10^{-34} \, J \, s, c = 3 \times 10^8 \, m \, s^{-1}, N_A = 6.022 \times 10^{23} \, mol^{-1})$

Calculate $(a)$ wavenumber and $(b)$ frequency of yellow radiation having wavelength $5800 \, \mathring{A}$.

How is an electron observed? Explain.

The wavelength of $X$-ray is $1.5 \times 10^{-10} \ m$. Find the frequency.