Why is the absorption wavelength same as the emission wavelength?

Question

Why is the absorption wavelength same as the emission wavelength?

Answer 1

It isn't.  A material that absorbs light of a particular wavelength does not reflect that same wavelength.

Answer 2

Absorption and emission wavelengths are equal when exactly the same two states are involved in each process, which is to say that there is no relaxation after absorption. This is generally only true for atoms because most molecules will rearrange their atoms to find a more stable geometry in the excited state (after absorption, but before fluorescence/emission).