What it does
A leaf does not use every colour of sunlight equally. Its pigments soak up some colours and let others bounce off. This page draws the absorption spectrum of the three main groups of leaf pigments, chlorophyll a, chlorophyll b and the carotenoids, against the colours of the rainbow from 400 nm (violet) to 700 nm (red), and on top of them the action spectrum: how fast photosynthesis goes in each colour.
Slide or drag the marker across the graph to pick a colour. The page tells you how strongly each pigment absorbs it and how fast photosynthesis would go, as a percentage of the best colour. At Class 11 level it also works out the energy of one photon of that colour and replays Engelmann’s experiment, where bacteria that love oxygen crowd around the parts of an alga lit by blue and red light.
It fits Class 10 Science (life processes: photosynthesis and chlorophyll) and Class 11 Biology (photosynthesis in higher plants: the pigments, absorption and action spectra). See how light, carbon dioxide and temperature limit the rate in photosynthesis lab, follow what the absorbed light does next in z-scheme stepper, and see how white light splits into these colours in prism and dispersion.
How to use it
- Choose Class 10 for the main idea, or Class 11 for the photon energy line and Engelmann’s experiment.
- Drag across the graph, or use the Colour of light slider.
- Tick or untick each curve to compare them one at a time. The dashed black curve is the rate of photosynthesis.
- Read the big line and the four numbers under it.
- At Class 11, watch the bacteria gather, then press Scatter the bacteria to start them again.
Worked example
Put the marker at 450 nm (blue). Chlorophyll b is near its tallest peak, the carotenoids absorb strongly and chlorophyll a absorbs a fair amount, so the page says the light is strongly absorbed, with photosynthesis at about 66 % of its best. One photon of 450 nm light carries E = hc/λ = (6.626 × 10⁻³⁴ × 2.998 × 10⁸)/(450 × 10⁻⁹) = 4.41 × 10⁻¹⁹ J, which is 2.76 eV.
Now slide to 550 nm (green). Every pigment curve is close to zero, the light is hardly absorbed and mostly reflected, and photosynthesis drops to about 9 % of its best. That reflected green light is what you see when you look at a leaf.
At 662 nm (red) chlorophyll a reaches its red peak and the rate climbs back to about 69 %. A red photon there carries only 1.87 eV, less than the blue one, but it still drives photosynthesis.
Tips
- An absorption spectrum belongs to one pigment; an action spectrum belongs to the whole process. When the two match, it is good evidence that those pigments do the work.
- Chlorophyll a is the main pigment; b and the carotenoids are accessory pigments that catch extra colours and pass the energy on.
- The rate never quite reaches zero in green, because light bounces around inside a real leaf and some of it is caught in the end.
FAQ
Are these real measured curves? No. They are smooth teaching curves built from bell shapes at the usual peak positions, so the shapes are right but the exact heights are rounded.
Why are autumn leaves yellow and orange? In autumn many trees break down their chlorophyll; the carotenoids, which absorb only blue, are left, so the leaves reflect yellow, orange and red.
What did Engelmann’s bacteria show? They gathered where the alga gave off the most oxygen, in blue and red light, which drew an action spectrum without any instruments.
Where the numbers come from
- Peak positions of chlorophyll a (about 430 and 662 nm), chlorophyll b (about 453 and 642 nm) and carotenoids (about 420 to 480 nm), and the idea that the action spectrum follows the absorption spectra: general plant biology. The curves are fibub's own sums of Gaussian bands with rounded, made-up heights and widths; they are not copied or measured spectra.
- Photon energy E = hc/λ with h = 6.62607015 × 10⁻³⁴ J s and c = 299 792 458 m/s (exact SI values) and e = 1.602176634 × 10⁻¹⁹ C.
- Engelmann's 1882 experiment with a filamentous alga and aerobic bacteria: history of science. The bacteria animation is a simple random-walk model, not data.
- Checked in the tests: the model's peaks fall at the stated wavelengths, and E at 450 nm = 2.755 eV (hand calculation 1239.84/450). Teaching model only. Nothing you enter is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.