Light from the Sun is a mixture of photons of different energies. When passed through a transparent prism in an instrument called a spectroscope, photons separate from on e another according to their energies, forming the electromagnetic spectrum.

Note that for photosynthesis to occur, light from the sun is required. Clusters of photosynthetic pigments, called photosystems, which are embedded in thylakoid membranes, absorb photons of particular wavelengths to perform photosynthesis. This was discovered by T.W. Engelmann.

Engelmann's Experiment

In 1882, the German botanist T.W. Engelmann performed an ingenious experiment in which he used the alga Spirogyra to determine whether all colours of the visible spectrum carried out photosynthesis equally well. Engelmann placed a triangular glass prism between the light source and the stage of a microscope. This setup caused the white light to spread into the colours of the spectrum as it passed through a slide on which he placed a sample of Spirogyra. He chose Spirogyra because it is a filamentous micro├Ârganism that possesses a long, spiral chloroplast throughout its length.

Engelmann carefully aligned the Spirogyra filament so that it was exposed to different colours (wavelengths) of light across its length. To determine whether photosynthesis occurred throughout the filament's length, he added aerobic bacteria to the slide, realizing that these would accumulate wherever oxygen was being produced (therefore, photosynthesis was occurring). After a period of time, he found that the bacteria accumulated in areas where the filament was exposed to red and blue-violet light, with very few bacteria gathering in the area illuminated with green light.
Image of Engelmann's experiment, otherwise known as the action spectrum for photosynthesis.

Determining the absorption spectrum of chlorophyll

a) Boil water
b) Place a leaf in the boiling water. Wait for 3 min
Why? To break the cell wall/membrane
c) Remove the leaf from the water and place it in a beaker containing alcohol
d) Place the beaker of alcohol in the beaker of water. Let it heat for approx 5 min.
e) Remove beaker of water from the hot plate. Remove the beaker of alcohol from the water. Pour a small amount of alcohol from the beaker into another beaker and dilute it with alcohol (or water).
Pour a small amount of the diluted mixture into a cuvette. Place cuvette in the spectrophotometer.
f) Record values. Plot values on a graph.

If anyone would like to put up the graph we generated on Friday, please do so. Unfortunately, I dunno how to reproduce this graph on Excel.

Compare the graph to the following spectra:

Absorption spectrum of chlorophyll a and b
Chlorophylls a and b absorb photons with energies in the blue-violet and red regions of the spectrum and reflect or transmit those with wavelengths between about 500 nm and 600 nm that our eyes see as green light. This is why most photosynthesizing organisms look green in white light. Comparison of the absorption and action spectra of a plant shows a close correspondence between the two, indicating that most of the wavelengths of light absorbed by chlorophylls are used in photosynthesis.
For more info on accessory pigments, see pages 152-153 in your textbook.
The sites listed below also offer basic info about accessory pigments.
http://ghs.gresham.k12.or.us/science/ps/sci/soph/energy/photosyn/pigments.htm
http://www.ucmp.berkeley.edu/glossary/gloss3/pigments.html

Plant pigments absorption spectrum
When the absorption spectra of chlorophylls a and b are combined with those of the accessory pigments, the absorption range covers almost the entire visible spectrum. The wavelengths from 400 nm and 700 nm are called photosynthetically active radiation (PAR) because, in general, these wavelengths support photosynthesis.