An absorption spectrum is a graph showing how much light a pigment absorbs at each wavelength. It is measured by passing light of different wavelengths through a pigment extract in a spectrophotometer (or a colorimeter with filters).
• Chlorophyll a has two main peaks, in the blue (about 430 nm) and red (about 660 nm) parts of the spectrum.
• Chlorophyll b has peaks at slightly different wavelengths, closer together, extending absorption in the blue and orange-red.
• Carotenoids absorb mainly blue-violet light, from about 400 to 500 nm.
• Very little light is absorbed between about 500 and 560 nm, the green part of the spectrum.An action spectrum is a graph of the rate of photosynthesis at each wavelength, measured for example as the rate of oxygen production. The action spectrum closely matches the combined absorption spectrum of the pigments. This is evidence that the light absorbed by these pigments is the light used in photosynthesis.The action spectrum is higher in the blue-green region than chlorophyll a alone would predict, because accessory pigments absorb light there and pass the energy on.A classic demonstration was carried out by Theodor Engelmann in 1882. He shone a spectrum of light onto a filament of green alga surrounded by bacteria that move towards oxygen. The bacteria gathered most densely where the alga was lit by red and blue light, showing that most oxygen, and so most photosynthesis, was produced there.
• Chlorophyll a has two main peaks, in the blue (about 430 nm) and red (about 660 nm) parts of the spectrum.
• Chlorophyll b has peaks at slightly different wavelengths, closer together, extending absorption in the blue and orange-red.
• Carotenoids absorb mainly blue-violet light, from about 400 to 500 nm.
• Very little light is absorbed between about 500 and 560 nm, the green part of the spectrum.An action spectrum is a graph of the rate of photosynthesis at each wavelength, measured for example as the rate of oxygen production. The action spectrum closely matches the combined absorption spectrum of the pigments. This is evidence that the light absorbed by these pigments is the light used in photosynthesis.The action spectrum is higher in the blue-green region than chlorophyll a alone would predict, because accessory pigments absorb light there and pass the energy on.A classic demonstration was carried out by Theodor Engelmann in 1882. He shone a spectrum of light onto a filament of green alga surrounded by bacteria that move towards oxygen. The bacteria gathered most densely where the alga was lit by red and blue light, showing that most oxygen, and so most photosynthesis, was produced there.