Conjugation and Light

Absorbtion of UV light or light from the visible region of the electromagnetic spectrum can promote an electron into its excited state. Without going into MO theory here you can know that some of these transitions are more favored and some have small enough energy barriers that they can be promoted by longer wavelengths. Conjugated dienes have a sufficiently small barrier such that they can transition to excited state (absorb) in the UV and Visible regions.
Allowing our skin to absorb UV rays from the sun puts us at higher risk for skin cancer. The active ingredients in sunblock absorb in the UV region, acting as a shield for our skin. If the suns rays are absorbed by a compound on top of our skin, they will not be absorbed by our skin. A popular active ingtredient for sunblocks is PABA or para-amino benzoic acid. All of the double bonds of the aromatic ring are conjugated, in addition the carbonyl is in conjugation with the aromatic ring. Another active ingredient that has come on the market more recently is Parsol. Parsol is said to block more UVA rays. It takes 1ooX more UVA light to cause the same damage as UVB light, but over the course of your lifetime, it is probably prudent to protect yourself from both. Parsol contains two ketones that are in conjugation with two para substituted aromatic rings.

As you increase the amount of conjugated pi bonds the energy between ground state and excited state is reduced, the absorption is reduced to lower wavelengths in the visible region. When a compound absorbs in the visible region it takes on the color of the wavelengths it does not absorb. Beta-carotene has 11 conjugated double bonds and has a maximum absorbance at 454 nm, the blue region. White light that appears orange once blue has been removed. Beta-carotene gives carrots and many other vegetables their color. Indigo absorbs at the other region of the visible spectrum, around 675 nm. If it is absorbing red and yellow, it will appear blue under white light.