what changes takes place in the atoms that account for the observed spectrum?

Answer 1

Hi. Electrons absorb photons which cause the electrons to jump to a higher, more energetic level. When they jump back they emit a photon at a characteristic frequency.

Answer 2

When you can see the spectrum of colours (that is the full range), it is the splitting up of white light particles., usually because of reflection, such as that of a rainbow or prism, which is called total internal reflection