Light absorption and pigments

The big idea: Photosynthesis is powered by light, but a leaf does not use all of the light that lands on it.

The green pigment chlorophyll absorbs some colours of light strongly and reflects others.

Chlorophyll absorbs blue and red light well, but it reflects green light back to your eye — which is exactly why leaves look green.

Pigment

A coloured molecule that absorbs some wavelengths (colours) of light and reflects others. The colour you see is the light the pigment does NOT absorb.

Chlorophyll

The main green pigment of plants. It absorbs blue and red light strongly and reflects green light, so it appears green.

Wavelength

What gives light its colour. Visible light runs from short-wavelength blue/violet (~400 nm) through green (~550 nm) to long-wavelength red (~700 nm).

Absorption spectrum

A graph showing how much light a pigment absorbs at each wavelength. For chlorophyll it has two peaks — one in the blue, one in the red — and a dip in the green.

The colour you see is the light it does NOT use: A leaf looks green because green is the colour chlorophyll reflects, not the colour it absorbs.

The useful light — the light that actually powers photosynthesis — is the blue and red that chlorophyll soaks up and you never see.

If chlorophyll absorbs blue and red light, then those should be the colours that drive photosynthesis fastest.

Scientists check this by comparing two graphs: the absorption spectrum (which colours the pigment absorbs) and the action spectrum (which colours make photosynthesis go fastest).

Absorption spectrum vs action spectrum: The absorption spectrum plots how much light a pigment absorbs at each wavelength.

The action spectrum plots the rate of photosynthesis at each wavelength.

The two graphs have almost the same shape — both peak in the blue and red and dip in the green.

They match because absorbed light is the energy source: the wavelengths the pigment absorbs are the wavelengths that power photosynthesis.

Accessory pigments fill the gaps: Chlorophyll absorbs blue and red well but misses some wavelengths in between.

Plants also contain accessory pigments, such as carotenoids (orange and yellow). These absorb some of the blue-green light that chlorophyll cannot, and pass the energy on to chlorophyll.

The result: the plant captures a wider range of wavelengths than chlorophyll could on its own — so it loses less of the available light energy.

A memory hook: Absorbtion spectrum = what a pigment absorbs. Action spectrum = the action (rate) of photosynthesis.

They line up because a plant can only use light that is first absorbed — no absorption, no action.

How this is tested: Photosynthesis spectra are a data-question favourite. On Paper 1B or Paper 3 you are often given an absorption or action spectrum and asked to read it — for example, identify the colour the pigment absorbs least (green) or the wavelength where photosynthesis is fastest.

A common Distinguish question asks for the difference between an absorption spectrum and an action spectrum, and why the two curves match.

Chromatography of a leaf extract is another data skill: separating the pigments shows there is more than one (different colours / different Rf values), which is why a leaf can absorb a range of wavelengths.

✓ Why this scores full marks: It uses the data (the low point at ~550 nm), not just memorised facts, and it connects three ideas: low absorption of green → green is reflected → reflected green is the colour we see.

A frequent slip is saying the leaf 'absorbs green' — it is the opposite: the leaf reflects the green it does not absorb.