Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light. It also reflects green-yellow light, and as such contributes to the observed green color of most plants.
Both chlorophyll A and chlorophyll B are molecules that the plant relies on to utilize the power of light in its life cycle. The two molecules differ by only a small composition sidechain; chlorophyll A is -CH3, and in chlorophyll B it is CHO. The process that chlorophyll utilizes light is known as photosynthesis.
Carotenoids cannot transfer sunlight energy directly to the photosynthetic pathway, but must pass their absorbed energy to chlorophyll. For this reason, they are called accessory pigments. One very visible accessory pigment is fucoxanthin the brown pigment which colors kelps and other brown algae as well as the diatoms.