What happens when light enters a molecule? 

As we know radiation in the UV-visible region can promote an electron to a higher energy orbital.

The energy taken in or absorbed by a molecule is directly related to the amount of energy (DE) required to promote an electron from one orbital to a higher energy orbital:

However, UV-visible spectroscopy is particularly applicable to CONJUGATED SYSTEMS e.g. b-carotene: 

b- Carotene is known as a chromophore. A chromophore absorbs in the visible and UV regions, but if it absorbs in the visible region, the molecule is seen to be coloured. Dyes usually have extended conjugated systems as the conjugation causes the p-p* transition (the transition between the filled and unfilled orbital) to be reduced in energy. Visible radiation is lower in energy than UV radiation and is capable of promoting an electron to the excited state, thereby conferring colour.

Generally the more conjugation in the molecule, the lower the energy required to promote an electron. This is because the gap between filled and unfilled orbitals is small and therefore not as much energy will be needed to promote an electron.