Here is a small puzzle with a satisfying answer. Leaves are green — but green is the one colour of light a leaf is throwing away. An absorption spectrum shows you exactly which colours it keeps instead.
📚 What you need to know
White light is a mixture of all the visible wavelengths, roughly 400 nm (violet) to 700 nm (red).
An absorption spectrum is a graph of how much light a pigment absorbs at each wavelength.
Chlorophylls absorb strongly in the blue-violet and red regions.
Carotenoids absorb mainly in the blue-violet region.
Green light is poorly absorbed and mostly reflected, which is why leaves look green.
The chemical structure of each pigment determines which wavelengths it can absorb.
Absorbed light excites electrons in the pigment, and that starts the chain of reactions of photosynthesis.
What the graph is showing
Shine each wavelength of light in turn at a solution of one pigment, and measure how much is absorbed. Plot absorbance against wavelength and you have an absorption spectrum for that pigment.
Each pigment gives a different shape, because each has a different chemical structure. A pigment can only absorb light whose energy matches what its electrons can take up — so structure decides the peaks.
Because the peaks do not line up exactly, the pigments cover between them a wider slice of the spectrum than any one of them could alone.
Reading the graph
Chlorophyll a has its main peak in the blue-violet, around 430 nm, and a second in the red, around 660 nm.
Chlorophyll b peaks slightly further along in each region, near 450 nm and 640 nm.
Carotenoids peak in the blue-violet only, roughly 450–480 nm, and absorb almost nothing in the red.
All three absorb very little between about 500 and 600 nm — the green and yellow part of the spectrum.
Carotenoids are called accessory pigments. They capture wavelengths the chlorophylls handle badly and pass the energy on, which widens the range of light the plant can actually use.
Why leaves are green
A pigment shows you the light it rejects. Blue and red are absorbed and used; green is largely reflected, so green is what reaches your eye.
This is the single most counter-intuitive idea in the topic, and examiners know it. The colour you see is the colour being wasted.
What absorbed light actually does
Light is not just soaked up and stored as heat. When a pigment molecule absorbs a photon, the energy raises one of its electrons to a higher energy level — the electron is excited.
That excited electron is then passed on, triggering the series of reactions that make up photosynthesis. By the end of the chain, the light energy has been transformed into chemical energy held in glucose.
Chain it like this in an answer: light absorbed → electrons excited → electrons transferred → reactions of photosynthesis → chemical energy in glucose. Examiners reward the sequence, not just the endpoints.
Worked examples
WORKED EXAMPLE
Using the absorption spectra, explain why a plant grown under green light alone would grow poorly. [3]
Step 1: read the graph at those wavelengths
Around 500–600 nm all the pigments show very low absorbance.
Step 2: say what that means for energy
Most green light is reflected, so little energy is absorbed by the pigments.
Step 3: link to the plant
Fewer electrons are excited, so the rate of photosynthesis and glucose production is low.
Little light absorbed means little energy converted, so growth is slowquote a wavelength range from the axis — it turns a vague answer into a specific one
WORKED EXAMPLE
Suggest an advantage to a plant of containing carotenoids as well as chlorophylls. [2]
Step 1: compare the spectra
Carotenoids peak at slightly different wavelengths from the chlorophylls.
Step 2: state the benefitwider range of wavelengths absorbed → more light energy capturedMore of the available light can be used, raising the rate of photosynthesis“absorbs more light” alone is thin — say it absorbs a wider range
WORKED EXAMPLE
A student states: “Chlorophyll is green, so it absorbs green light.” Explain the error. [2]
Step 1: separate the two ideas
The colour we see is the light reflected, not the light absorbed.
Step 2: give the correct version
Chlorophyll absorbs mainly blue-violet and red, and reflects green.
It looks green because green is the wavelength it does not absorbthe absorption spectrum shows a trough, not a peak, in the green region
💡 Exam tip
Quote wavelengths in nm when describing a spectrum. “Peaks at about 430 nm and 660 nm” beats “peaks at each end”.
Say blue-violet rather than just “blue” — it matches the mark scheme wording.
Remember the y-axis is absorbance, not rate. Rate belongs to the action spectrum on the next page.
For any “why is a leaf green” question, use the words reflected and absorbed in the same sentence.
Mention chemical structure if asked why different pigments absorb different wavelengths.
Describe carotenoids as accessory pigments that broaden the range absorbed.
⚠ Common mix-up
Thinking a green pigment absorbs green light. It reflects it. This is the classic trap on this page.
Saying no green light is absorbed at all. A little is — the graph shows a trough, not zero.
Confusing an absorption spectrum with an action spectrum. One measures absorbance, the other measures rate.
Claiming carotenoids absorb red light. They have no red peak; only the chlorophylls do.
Treating chlorophyll as a single pigment. There are at least two, with slightly different peaks.
Saying light is “stored” in the pigment. It excites electrons, which are then passed on.
Up next: Absorption & Action Spectra (Skills) — measuring the rate of photosynthesis at each wavelength, and seeing how closely it tracks the absorbance you have just plotted.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
