What is the wavelength of light emitted when the electron in hydrogen atom undergoes transition from an energy level with n = 6 to an energy level with n = 3 ?

What is the wavelength of light emitted when the electron in a hydrogen atom undergoes trnsition from an energy level with n = 4 to an energy level with n = 2 ?

What is Fermi energy level ?

What will be the wavelength of an electron of energy 500 eV?

What are energy levels or shells ?