The part of solar energy that drives photosynthesis is visible light, specifically wavelengths between 400 and 700 nanometers. Scientists call this range photosynthetically active radiation, or PAR. It corresponds almost exactly to what your eyes perceive as the colors violet through red. Within that band, red and blue light do the heaviest lifting, while green light contributes less efficiently.
Why 400 to 700 Nanometers Matters
Sunlight contains a broad spectrum of energy, from short-wavelength ultraviolet rays to long-wavelength infrared heat. Plants can only harness a slice of that spectrum. The 400 to 700 nm window works because photons in this range carry just the right amount of energy to excite the pigment molecules inside plant cells without destroying them. Shorter wavelengths (ultraviolet) carry too much energy and damage cellular machinery. Longer wavelengths (infrared) carry too little to trigger the chemical reactions photosynthesis depends on.
This 400 to 700 nm band represents roughly 43 to 50% of total solar radiation reaching Earth’s surface. So even before any biological inefficiency enters the picture, plants can only access about half the sun’s energy output. The theoretical maximum efficiency for converting solar energy into plant biomass is about 4.6% for most common crops and 6% for plants like corn and sugarcane that use a more advanced carbon-fixing pathway.
Red and Blue Light Are the Most Efficient
Not all visible wavelengths contribute equally. Red light (around 620 to 700 nm) and blue light (around 420 to 470 nm) are absorbed most strongly by chlorophyll, the primary pigment in leaves. Red photons are particularly efficient because they carry enough energy to drive the photosynthetic reaction without wasting much as heat. Blue photons carry more energy than needed, so the excess is released as heat, making them slightly less efficient per photon, though still heavily used.
Green light (roughly 500 to 565 nm) is mostly reflected or transmitted by leaves, which is why plants look green to us. That said, green light isn’t useless. It penetrates deeper into thick leaf tissue and dense canopies, contributing modestly to photosynthesis in cells that red and blue light can’t reach.
How Pigments Capture Light Energy
Chlorophyll is the star player, but it doesn’t work alone. Plants contain accessory pigments called carotenoids that absorb in the blue-green region of the spectrum (roughly 400 to 550 nm) and pass that captured energy to chlorophyll. This handoff fills a gap in chlorophyll’s absorption range and boosts overall efficiency. Carotenoids serve a second critical role: they act as a safety valve, dissipating excess light energy that would otherwise damage the cell. This protective function is why leaves turn yellow and orange in autumn, as chlorophyll breaks down and reveals the carotenoids underneath.
Inside each leaf cell, these pigments are organized into two cooperative systems called photosystem I and photosystem II. When a photon strikes chlorophyll in photosystem II, it excites electrons to a higher energy state. Those energized electrons are pulled away from the chlorophyll and passed through a chain of molecules. The energy lost at each step is used to build the chemical fuel (ATP and NADPH) that powers sugar production. To replace the lost electrons, photosystem II splits water molecules, releasing oxygen as a byproduct. Photosystem I absorbs light peaking around 700 nm, while photosystem II absorbs best around 680 nm, and both must operate together in sequence for the full process to work.
The Surprising Role of Far-Red Light
Light just beyond the visible red range, called far-red light (700 to 750 nm), was long considered useless for photosynthesis. It’s poorly absorbed by leaves and produces very low photosynthetic output when used alone. But research dating back to Robert Emerson’s work in the 1950s revealed something unexpected: when far-red light is combined with shorter-wavelength light, total photosynthesis exceeds the sum of what each wavelength produces separately. This synergistic boost is known as the Emerson enhancement effect.
The explanation comes down to balance. Shorter wavelengths (400 to 670 nm) tend to over-excite photosystem II relative to photosystem I. Far-red light does the opposite, preferentially driving photosystem I. Since the two photosystems work in series, like a two-step assembly line, unbalanced excitation creates a bottleneck. Adding far-red light to shorter wavelengths restores balance between the two systems, and the whole chain runs more smoothly. This is why some modern greenhouse lighting systems include far-red LEDs alongside red and blue ones.
Ultraviolet Light: Energy Plants Must Defend Against
Ultraviolet radiation (below 400 nm) is not used productively in photosynthesis and actively damages the machinery that makes it work. UV-B rays are especially harmful. Their primary target is the manganese cluster involved in water splitting, the very first step of converting light energy into chemical energy. UV-B also degrades key proteins in photosystem II and impairs the enzyme responsible for fixing carbon dioxide into sugar. Plants defend against this damage with sunscreen-like compounds in their outer cell layers, but intense UV exposure still reduces photosynthetic capacity.
Physiologically, plants do respond to a broader range than just 400 to 700 nm. UV-A light (315 to 400 nm) triggers protective and developmental responses, and far-red light influences flowering, stem elongation, and shade avoidance. But for the core energy-harvesting reaction of photosynthesis, the 400 to 700 nm band of visible light remains the critical input.
Measuring Light for Plant Growth
If you’re growing plants indoors or in a greenhouse, the relevant measurement isn’t brightness as your eyes perceive it (measured in lumens) but the number of photons in the 400 to 700 nm range hitting a given area each second. This is called photosynthetic photon flux density, or PPFD, measured in micromoles of photons per square meter per second (μmol/m²/s). A shade-adapted plant may reach its maximum photosynthetic rate at around 200 μmol/m²/s, while full-sun crops need several times that. When choosing grow lights, PPFD in the red and blue ranges matters far more than raw wattage or perceived brightness.