The absorption spectrum is an indication of the wavelength of light absorbed by each pigment.
It is measured using a spectrophotometer.
A spectrophotometer is a machine that sends light through pigments and measures the fraction of light transmitted by each wavelength.
Hence, the absorption spectrum is the graph plotting light absorption versus wavelength of that light.
The absorption spectra of chloroplast pigments provide an insight into the relative effectiveness of different wavelengths of light for driving photosynthesis.
Chlorophyll absorbs more of blue-violet wavelength than the red.
The absorption spectrum is chiefly blue and red for chlorophyll.
Chlorophyll a absorbs more of red wavelength than chlorophyll b.
Chlorophyll b absorbs more of blue wavelength than chlorophyll a.
Action spectrum confirms the effectiveness of red wavelength.