Plants get their light energy from the sun. Solar radiation in the 400 to 700 nanometer wavelength range, known as photosynthetically active radiation (PAR), provides the energy that drives photosynthesis. This is essentially the same band of light that your eyes perceive as visible color, from violet and blue on one end to red on the other. Plants can also use artificial light sources like LEDs and fluorescent bulbs, as long as those lights deliver the right wavelengths and intensity.
Which Colors of Light Plants Actually Use
Not all colors of sunlight are equally useful to a plant. The pigments responsible for capturing light, chlorophyll a and chlorophyll b, absorb most strongly in two specific bands: blue light (400 to 500 nm) and red light (600 to 700 nm). Chlorophyll a has a major absorption peak around 660 nm in the red range, while chlorophyll b peaks near 653 nm. Both pigments also absorb blue light strongly. This selective absorption is why most leaves appear green: the green wavelengths (500 to 600 nm) are the ones the plant reflects rather than absorbs.
Plants have backup pigments too. Carotenoids, the same compounds that give carrots and autumn leaves their orange and yellow color, absorb light mainly between 400 and 500 nm. They pass that captured energy along to chlorophyll, effectively widening the slice of sunlight a plant can harvest.
There is also growing evidence that far-red light (700 to 750 nm), which sits just beyond what we typically see as red, contributes more to photosynthesis than scientists once thought. Far-red photons were traditionally excluded from the definition of PAR because their energy yield drops off above 700 nm. But recent work has shown that far-red light can be just as efficient as standard PAR on a per-photon basis when it’s part of a broader light spectrum, particularly because it helps balance the workload between the two photosystems inside the chloroplast.
How a Leaf Captures a Photon
The actual energy capture happens inside chloroplasts, tiny organelles packed into leaf cells. Within each chloroplast is a system of flattened, disc-shaped sacs called thylakoids. The thylakoid membranes contain large protein complexes called photosystems, each built from two components: an antenna complex and a reaction center.
The antenna complex is a cluster of hundreds of pigment molecules, mostly chlorophyll, arranged to act like a satellite dish. When a photon of sunlight strikes any one of these pigment molecules, it bumps an electron to a higher energy state. That burst of energy hops rapidly from one pigment molecule to the next through a process called resonance energy transfer, funneling inward until it reaches a special pair of chlorophyll molecules sitting at the heart of the reaction center.
This special pair acts as a one-way trap. The moment the energy arrives, it excites an electron that is immediately handed off to a chain of neighboring molecules called electron acceptors. The electron doesn’t bounce back. Instead, it moves down an electron-transport chain embedded in the thylakoid membrane, releasing energy at each step. This is conceptually similar to how a ball rolling downhill loses potential energy. The energy released along this chain is used to build two key molecules: ATP (the cell’s universal energy currency) and NADPH (an electron carrier). Together, these molecules power the next stage of photosynthesis, where carbon dioxide from the air is assembled into sugar.
How Efficient the Process Really Is
Despite how elegantly the system works, plants convert a surprisingly small fraction of the sunlight hitting them into stored energy. The theoretical maximum efficiency is about 4.6% for most common plants (known as C3 plants, which include wheat, rice, and most trees) and around 6% for C4 plants like corn and sugarcane, which have an extra biochemical step that reduces energy waste.
In practice, real crops fall well short of even these modest ceilings. Across a full growing season, the highest reported conversion efficiencies are roughly 2.4% for C3 crops and 3.7% for C4 crops. Brief peak periods can push those numbers to 3.5% and 4.3%, respectively, but sustained performance is lower. The rest of the solar energy is lost to reflection, heat, wavelengths the plant can’t use, and inefficiencies in the chemical reactions themselves.
How Artificial Light Substitutes for the Sun
Plants don’t care whether photons come from the sun or a light bulb. What matters is wavelength and intensity. This is why indoor farming and grow lights work. Modern LEDs are particularly effective because they can be tuned to emit specific wavelengths that match what chlorophyll absorbs best.
Red LEDs in the 650 to 665 nm range align almost perfectly with chlorophyll’s peak absorption, making them highly efficient at driving photosynthesis. But red light alone isn’t enough. Adding blue light (around 400 to 500 nm) produces better overall growth, and the combination of red and blue LEDs supports higher photosynthetic activity than either color on its own. As little as 15 micromoles per square meter per second of blue light is enough to prevent the stretched, pale growth that plants develop when they don’t get adequate blue wavelengths.
Green light plays a smaller but real role. Small amounts of green light, up to about 24% of the total spectrum, can actually enhance growth in some species, likely because green wavelengths penetrate deeper into thick leaf canopies where red and blue light has already been absorbed. However, when green light exceeds about 50% of the total, plant growth declines.
Light intensity also matters. If the light is too dim, photosynthesis can’t keep up with the plant’s energy needs, and growth stalls. If it’s too intense, the excess energy generates harmful oxygen radicals inside the leaf and the photosynthetic machinery temporarily shuts down to protect itself, a response called photoinhibition. Every plant species has a sweet spot, and getting both wavelength and intensity right is the key to replacing sunlight effectively.