What is the Difference Between Action Spectrum and Absorption Spectrum

The key difference between action spectrum and absorption spectrum is that the action spectrum describes the wavelengths that are responsible for carrying out photosynthesis, whereas the absorption spectrum shows the wavelengths that undergo absorption when light is passed through a certain sample.

A spectrum is a band of colors produced due to the separation of light components through different degrees of refraction depending on the wavelength. Action spectrum and absorption spectrum are two such types of spectra.

CONTENT

1. Overview and Key Difference
2. What is an Action Spectrum
3. What is an Absorption Spectrum
4. Action Spectrum vs Absorption Spectrum in Tabular Form
5. Summary – Action Spectrum vs Absorption Spectrum

What is an Action Spectrum?

Action spectrum can be described as a graph that shows the rate of biological effectiveness plotted against the wavelength of light. This spectrum is related to the absorption spectrum in different systems. Moreover, it tends to describe the inverse quantity of light that is needed to keep a constant response. These spectra are very rare in describing the level of biological activity because biological responses are typically nonlinear with intensity.

Further, the action spectra are written as a unit-less response where there is a peak response that is important in distinguishing whether an action spectrum refers to the log-photons amount at each wavelength or to the spectral power.

Besides, the action spectrum explains the wavelength that is most effectively used in a specific chemical reaction. Typically, some reactants can use particular wavelengths of light effectively in completing the reactions. E.g. chlorophyll is efficient when it uses red and blue regions compared to the green region of the spectrum of light when carrying out photosynthesis. In this context, the spectrum graph shows spikes in red and blue color representing wavelengths.

What is an Absorption Spectrum?

An absorption spectrum is a type of spectrum containing the frequencies of light transmitted with dark bands when the electrons in the ground state absorb energy to reach a higher energy state. In other words, these spectra are formed when atoms are absorbing energy. There are three types of absorption spectra named as continuous absorption spectrum, line absorption spectrum, and band absorption spectrum.

Simply, the absorption spectrum shows the change in absorbance of a sample in the form of a function of the wavelength of incident light. We can measure and detect the absorption spectrum using a spectrophotometer.

According to the nature of the chemical substance or chemical element, some radiation types are absorbed by them when light passes through the sample. It shows a dark line pattern where the colored lines are in between the dark lines. It is the opposite pattern of the emission spectrum.

What is the Difference Between Action Spectrum and Absorption Spectrum?

The key difference between action spectrum and absorption spectrum is that the action spectrum describes the wavelengths that are responsible for carrying out photosynthesis. Meanwhile, the absorption spectrum shows the wavelengths that undergo absorption when light is passed through a certain sample.

The below infographic presents the differences between action spectrum and absorption spectrum in tabular form for side-by-side comparison.

Summary – Action Spectrum vs Absorption Spectrum

The action spectrum is a graph that shows the rate of biological effectiveness plotted against the wavelength of light. An absorption spectrum is a type of spectrum containing the frequencies of light transmitted with dark bands when the electrons in the ground state absorb energy to reach a higher energy state. The key difference between action spectrum and absorption spectrum is that the action spectrum describes the wavelengths that are responsible for carrying out photosynthesis. Meanwhile, the absorption spectrum shows the wavelengths that undergo absorption when light is passed through a certain sample.

Reference:

1. “Action Spectrum.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “Chlorophyll ab spectra-en” By Original: Daniele Pugliesi Vector: M0tty – This file was derived from Chlorophyll ab spectra2.PNG: (CC BY-SA 3.0) via Commons Wikimedia
2. “Absorption spectrum of few elements” By Almuazi – Own work (CC BY-SA 4.0) via Commons Wikimedia