Q.Describe Engelmann's prism-and-Cladophora experiment in some detail, and explain precisely what it revealed about the relationship between light absorption and photosynthetic activity.

[!TLDR]

Engelmann split light through a prism over an alga surrounded by O2-seeking bacteria; bacterial clustering in blue/red light showed the action spectrum tracks chlorophyll's absorption.

[!ANSWER]

Engelmann projected a light spectrum across a filamentous alga (Cladophora) in a suspension of aerobic bacteria; the bacteria clustered densely wherever O2 evolution (photosynthesis) was highest, revealing that this coincided with the blue and red regions -- the wavelengths chlorophyll absorbs most strongly.

In Engelmann's set-up, a beam of light was passed through a prism so that different colours fell on different parts of the same alga simultaneously. He then introduced motile, aerobic bacteria into the suspension, which are naturally attracted toward regions of higher O2 concentration. Wherever the alga was photosynthesising most actively, more O2 was released locally, and the bacteria congregated there in visibly greater numbers, acting as a living, real-time oxygen sensor without any chemical apparatus at all. Engelmann found the bacteria concentrated overwhelmingly in the blue and red parts of the spectrum, and were comparatively sparse in the green part -- a pattern matching chlorophyll's own absorption peaks precisely. This directly demonstrated that a pigment's absorption spectrum and the action spectrum (actual photosynthetic output by wavelength) closely correspond, providing the first strong evidence that chlorophyll is the pigment actually responsible for driving photosynthesis, rather than merely being present in photosynthetic tissue incidentally.

[!ANSWER]

Engelmann used a prism to split light across an alga in a bacterial suspension; the aerobic, O2-seeking bacteria clustered densest in the blue and red regions, matching chlorophyll's absorption peaks and showing that the action spectrum parallels chlorophyll's absorption spectrum.

Walk through the experimental logic step by step: prism splits light -> alga photosynthesises differently by wavelength -> bacteria detect and reveal the local O2 output.

Describing the bacteria as photosynthetic organisms themselves, rather than as an indicator/detector of the O2 the alga produced.