The energy stored in big molecules (such as carbohydrates) created via photosynthesis is derived in part through the light energy in photons. In order to tap into this energy, light must be absorbed by plants and other photosynthetic organisms.
As you know, visible light ranges in wavelength with colour:
Between 400-700 nm, light passes through several colours from violet to red. Pigments absorb some wavelengths more than others, just like anything else we see as coloured. For example, something appears yellow if it absorbs other colours like blue (500 nm) and red (700 nm) but reflects yellow (600 nm).
The two main classes of pigments in photosynthesis are chlorophyll of which there are multiple types (a, b, c, etc.) and carotenoids of which there are also multiple. The former are, surprise! green, while the latter are yellow, orange or red.
Their absorption spectra are different. Chlorophyll b for example, absorbs blue light excellently, as well as some orange light. Carotenoids only absorb blue light, with some towards the violet end of the spectrum as well as towards the green wavelengths.
Plants can make use of these multiple pigments to maximise their light absorption potential. Together, these pigments offer a range of 400-530 nm and 650-700 nm which is a total of 180 nm accessible wavelength values, out of 300 nm of visible light. That’s 60% of wavelengths. These are available as light for photosynthesis.
This information was discovered by looking at the action spectra of pigments, together with their absorption spectra. A tight correlation was found.
The photosynthetic rate (bold curve) covers all areas of wavelength corresponding to the absorption of three different pigments (faint curves). This confirms that the light absorbing property of the pigments is linked to the ability to carry out photosynthesis.
If a plant were to only have chlorophyll b (faint dark green curve), imagine what the action spectrum would look like! It would follow the chlorophyll b curve and overall provide a much narrower range of ability to photosynthesis compared with all three together. This is why having a variety of pigments is crucial.