Question

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

Answer 1

The equation you'll want to use is Bohr's formula: \[E = -2.178 x 10^{-18}(\frac{ 1 }{ n^{2}})\]

The letter n represents the energy level in your equation. You'll need to do it twice, once for each state. The change of the energy is what you'll use when you calculate \[E = hv\]

The h is Planck's constant, and the V (nu) is the frequency. From there you should be able to get lambda x nu = C.