Quantum mechanics in atoms question

[jazzchan]

The question is:
A hydrogen atom absorbs a photon and makes a transition from a 3d level to a 4f level. What is the wavelength of the absorbed photon ?

As i know 3d is n = 3, 4f is n = 4

and wavelength = hc/delta E

so the deltaE is E4 - E3 or E3 - E4 ??

thanks..

[turin]

I think that's right. You also need to know what the energy is (compared to ionization energy, for instance) at a given principle quantum number.

[JohnDubYa]

Just use the absolute value of E4 - E3, because all you are interested in is the AMOUNT of energy needed to make the transition, which is a positive value. So it makes no difference if you use E4 - E3 or E3 - E4, since the absolute value produces the same result.

Since this is hydrogen, then E_i = -13.6/n^2 (in eV).