Light and Types of Spectrum

Nature and units of light

Light is form of electromagnetic energy. Electromagnetic energy travels in the form of waves.

What is wavelength?

The distance between the crest of electromagnetic waves is called wavelength.

What is electromagnetic spectrum?

The entire range of radiations is known as electromagnetic spectrum. This spectrum includes wavelength of less than nanometer to more than a kilometer.

Types of Spectrum

There are three different types of spectrum are discussed:

  1. Visible Spectrum
  2. Absorption Spectrum
  3. Action Spectrum

1. Visible Spectrum

The narrow band of wavelength from 380-750nm of electromagnetic spectrum is most important for life. It is called visible spectrum. Its various colors are detected by human eye. The visible spectrum includes red, orange, yellow, green, blue, indigo and violet.

Light travels in the form of photons. The energy of photon is related to its wavelength. Shorter the wavelength, greater will be the energy. The radiations that derive energy are blue and red. These wavelengths are most effectively absorbed by the chlorophyll.

2. Absorption Spectrum

A plot (graph) showing absorption of light of different wavelength by a pigment is called absorption spectrum. Light absorb is measured by spectrophotometer.

Different pigments:

Light absorbed by the different pigments i.e. Chl a, Chl b and Caroteniods is plotted against wavelength. It gives absorption spectrum. Both Chlorophyll a and b show maximum absorption in the blue, violet, orange and red region of visible spectrum. Green and yellow and indigo light are lightly absorbed by chlorophyll. That is why chloroplast appear to be green.

Absorption Spectrum
Absorption Spectrum

Blue and red light are used by the green plants as a energy source for photosynthesis. Absorption of chlorophyll shows two peak at 430, 680 and 700 nm. Carotenoids absorb radiant energy between 449 and 478 nm. Xanthophylls show peak at 440 and 490 nm.

3. Action Spectrum

Graph showing relative effectiveness of different wavelength of light in photosynthesis. Action spectrum identifies the pigments actually involved in the photosynthesis. The two photosystem were discovered by action spectrum. The action spectra are measured by spectrographs. First Action Spectrum was designed by T.W. Engelman and it was of spirogyra.



Absorption spectrum is given by all the pigments like Chlorophyll a, b, carotene and xanthophylls. Action spectrum is given by only Chl a. It means chlorophyll a forms the reaction center.

There is great difference between absorption and action spectrum of Chl a. The peaks of action spectrum have more height and its valley are much narrow than absorption spectrum. It indicates that all other pigments are accessory which transfer energy to Chl a.

comparison between them

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