Why do spectra appear continuous despite their allowed transitions being discrete? So in my physical chemistry class we have been looking into radiation, selection rules, etc.

Taking into account the discrete levels of energy an electron can have, I cannot understand why fluorescence and absorption spectra of compounds have continuous lines instead of discrete values.

If electrons in a system can only have a select number of energies, why is it that these graphs show a continuum of absorption and emission?

I also have this problem with blackbody radiation, since it is a continuous line for any temperature.
You are right that absorption and emission are quantized processes, but UV-visible peaks represent a broad range of wavelengths because there are multiple vibrational states of their representative ground and excited electronic states.



In the case on my diagram, you have six different quantized electronic transitions with slightly different &Delta;E values. In reality, there are more energy levels than that, plus the rotations and vibrations are constantly changing the orbital energy levels as a result of interactions with the matrix, so that helps fill in the peaks. The peak shape can actually be changed by altering the temperature. This gives rise to high-resolution UV-Visible spectrophotometric techniques, which have a few specific applications.