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Questions on action spectrum and absorption spectrum appear in NEET and board exams, and many students treat them as the same graph. They are not, and the cleanest way to understand the difference is through a single experiment from the 1880s that used living bacteria as oxygen detectors. This post explains what Theodor Engelmann did, how to read his result, and the mistakes examiners look for.
Who was Theodor Engelmann?
The NCERT Class 11 Biology chapter on photosynthesis gives his dates as 1843-1909. A historical review in Photosynthesis Research, Drews (2005), describes him as someone "who had a creative life in music, muscle physiology, and microbiology". The same abstract credits him with developing a sensitive method for tracing the oxygen produced in photosynthesis by single-celled plants, using the movement of bacteria towards oxygen.
He published a description of that method in 1881, in Pflügers Archiv: "Neue Methode zur Untersuchung der Sauerstoffausscheidung pflanzlicher und thierischer Organismen", roughly, a new method for studying oxygen release by plant and animal organisms. In 1882 he published, in the same journal, "Ueber Sauerstoffausscheidung von Pflanzenzellen im Mikrospektrum", on oxygen release from plant cells in a microscope-sized spectrum.
What was the experiment?
NCERT describes it in four steps:
Using a prism, he split light into its spectral colours.
He shone this spectrum onto a filament of the green alga Cladophora, so different parts of the filament received different colours.
The alga sat in a suspension of aerobic bacteria. Aerobic bacteria need oxygen and move towards it, so they act as detectors for where oxygen is being released.
He looked to see where the bacteria gathered.
The result: the bacteria accumulated mainly in the regions lit by blue and red light. Fewer gathered under green light. In NCERT's words, "A first action spectrum of photosynthesis was thus described." It "resembles roughly the absorption spectra of chlorophyll a and b."
Why was this such a clever design?
In the 1880s there was no electronic oxygen sensor fine enough to measure output along a single algal filament. Engelmann used organisms whose behaviour already reported oxygen. The bacteria did the measuring, and the microscope let him see the result directly. The same review notes that he went on to study light responses in purple bacteria too. His 1883 paper on a purple bacterium he called Bacterium photometricum used organisms that react to light themselves.
Worked example: turning bacteria counts into an action spectrum
Here is how to think about the data. The counts below are sample numbers made up for practice, not Engelmann's readings. Suppose you counted bacteria near each colour band along the filament:
Violet: 20
Blue: 46
Green: 8
Yellow: 10
Orange: 18
Red: 42
Step 1. Rank the bands: blue (46) and red (42) are highest; green (8) is lowest.
Step 2. Interpret: more bacteria means more oxygen, which means faster photosynthesis. So blue and red light drive photosynthesis most, and green least.
Step 3. Compare with pigments: chlorophyll a and b absorb strongly in blue and red and weakly in green. The match suggests chlorophylls are the main pigments capturing light for photosynthesis.
Step 4. Notice the mismatch: green is low but not zero. NCERT explains that accessory pigments such as chlorophyll b, xanthophylls and carotenoids let a wider range of wavelengths be used, which is one reason the action spectrum does not overlap one-to-one with the absorption spectrum of chlorophyll a. Figure 11.3 in the chapter puts the two graphs side by side.
Common mistakes:
Saying the bacteria photosynthesise. In this experiment they are aerobic bacteria that respond to oxygen; the alga photosynthesises.
Saying green light is not used at all. It is used less, not never.
Drawing the action spectrum and the absorption spectrum as identical curves. They are similar in shape but not the same.
Writing that leaves look green because they absorb green light. They reflect and transmit much of it.
Where does Engelmann fit in the story of photosynthesis?
The NCERT history runs in order: Priestley shows plants restore air; Jan Ingenhousz shows light is needed and that green parts release oxygen; Julius von Sachs shows glucose is made in green parts; Engelmann shows which colours of light work; and Cornelius van Niel argues that the oxygen comes from water. Each step answers a narrower question than the one before. For the reactions themselves, see our guide to photosynthesis for NEET.
What can a student take from Engelmann's way of working?
Find a detector that already exists. He did not build an oxygen meter; he used bacteria that respond to oxygen.
Spread the variable out in space. By laying the whole spectrum along one filament, he tested every colour at the same time, under the same conditions.
Compare your result with something known. The value of his spectrum came from setting it beside the absorption of chlorophyll.
If the difference between the two kinds of spectrum still blurs, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can look up each graph on its own.
Next step: sketch an absorption spectrum for chlorophyll a and, on the same axes, an action spectrum for photosynthesis, then label one place where they differ.
