Plants do not absorb all types of light, but they use a much broader range than most people realize. The wavelengths that matter most for photosynthesis fall between 400 and 700 nanometers, a range known as photosynthetically active radiation (PAR). This covers visible light from violet through red. Outside that window, plants still detect and respond to ultraviolet and far-red light, but they largely ignore mid-infrared and radio waves.
Which Colors Drive Photosynthesis
The heavy lifters are red and blue light. Chlorophyll a, the primary photosynthetic pigment, absorbs most strongly at about 430 nm (blue) and 662 nm (red). Chlorophyll b peaks at 453 nm (blue) and 642 nm (red). Together, these two pigments capture the energy that powers sugar production in leaves. Plant leaves absorb roughly 90% of the red and blue light that hits them.
Plants also carry accessory pigments, mainly carotenoids and xanthophylls, that fill gaps in chlorophyll’s absorption range. These orange and yellow pigments absorb wavelengths in the 420 to 510 nm range and pass that energy along to chlorophyll. You see them most obviously in autumn, when chlorophyll breaks down and carotenoid colors are unmasked.
Green Light Does More Than You’d Think
The common explanation for why leaves look green is that they reflect green light and absorb everything else. That’s an oversimplification. Leaves actually absorb 70 to 80% of green light, with some species like lettuce absorbing closer to 50% at peak green wavelengths around 550 nm. The reason plants appear green is that green is the least absorbed color, so it’s the one most likely to bounce back to your eye. But “least absorbed” is not the same as “not absorbed.”
Green light also has a practical advantage in dense vegetation. Red and blue wavelengths drop off steeply as light filters through upper leaves, while green light penetrates much deeper into a canopy. This means lower leaves, which would otherwise sit in near-darkness, can still photosynthesize using the green wavelengths that made it through. Whether this meaningfully boosts whole-plant photosynthesis in real growing conditions is still debated, but green light is far from useless.
Far-Red Light and the Enhancement Effect
Far-red light sits just beyond the visible red range, roughly 700 to 800 nm. On its own, it drives photosynthesis poorly. But when combined with shorter wavelengths, something interesting happens: total photosynthesis exceeds the sum of what each wavelength achieves alone. This is called the Emerson enhancement effect, first discovered in the late 1950s.
The reason comes down to how the two photosynthetic systems inside a chloroplast work. Photosystem I and photosystem II operate in series, like a two-step assembly line. Visible light tends to over-excite photosystem II relative to photosystem I, creating a bottleneck. Far-red light preferentially drives photosystem I, balancing the workload. In shade-grown tomato leaves, adding 720 nm far-red light to a natural shade spectrum boosted photosynthetic efficiency by as much as 76%. Even sun-grown leaves showed a 46% enhancement when far-red light was added. This is why the standard PAR definition of 400 to 700 nm actually underestimates useful light in shaded, far-red-rich environments like forest understories.
Ultraviolet Light: Stress Signal, Not Fuel
UV light (10 to 400 nm) is not used for photosynthesis, but plants absolutely detect it and respond. Plants have a dedicated UV-B photoreceptor called UVR8 that senses wavelengths in the UV-B range (280 to 315 nm). When UV-B light hits this receptor, it triggers a cascade of changes: altered flowering time, shorter stems, more compact growth, and increased production of protective compounds like flavonoids and anthocyanins. These are the same pigments responsible for deep purple and red coloring in fruits and leaves.
At low doses, UV-B acts as a useful signal that helps plants build disease resistance and produce defensive chemicals. At high doses, it becomes damaging, breaking DNA strands and generating harmful reactive oxygen species faster than the plant can neutralize them. This is why plants at high altitudes or in the tropics often produce more UV-protective pigments than those grown indoors or at high latitudes.
Light Plants Use for Growth, Not Energy
Beyond photosynthesis, plants use light as information. Two families of light-sensing proteins handle most of this work. Phytochromes absorb red and far-red light and toggle between two forms: one activated by red light, the other deactivated by far-red. This system tells the plant whether it’s in full sun or shade, since shade is enriched in far-red light. The ratio of red to far-red controls seed germination, stem elongation, and the timing of flowering.
Cryptochromes sense blue light and regulate many of the same developmental processes, including when a plant flowers and how it responds to day length. Together, phytochromes and cryptochromes allow plants to track not just how much light is available but what kind, adjusting their growth strategy accordingly. A seedling under a forest canopy, for example, will stretch taller and allocate less energy to leaf width because its phytochromes detect a low red-to-far-red ratio, signaling competition from taller neighbors.
What Plants Truly Ignore
Mid-infrared, microwave, and radio wavelengths pass through or around plant tissue without triggering any known biological response. Infrared radiation above about 800 nm is primarily experienced as heat, and while temperature obviously affects plant growth, this isn’t light absorption in any photochemically meaningful sense. Plants have no known receptors or pigments tuned to these longer wavelengths.
So the short answer is that plants absorb most of the visible spectrum (red and blue strongly, green partially), respond to UV and far-red through specialized receptors, and ignore everything beyond that range. The textbook image of plants using only red and blue light dramatically understates how many wavelengths actually matter to a living leaf.