Action Spectrum vs Absorption Spectrum- Know the Differences

You can’t perform adhering activities effectively without knowing these spectrum differences exactly. However, a general confusion arises among learners to differentiate between multiple types of spectrum. The confusion gets deeper when it comes to the action spectrum vs absorption spectrum.

Learning about these spectrums is significant since the characteristic of the wavelength depends on it. If you fail to identify the difference, it may cause you a serious problem that you don’t want.

Tons of differences are there. You will find some similarities as well. Knowing these will let you use the spectrum properly and effectively. So let’s dive into the main comparison part.


Action Spectrum vs Absorption Spectrum

Action Spectrum vs Absorption Spectrum Comparison

To determine the separate uses of two spectrums along with advantages, it’s mandatory to know their specifications. Therefore, let’s know the distinction between action spectrum vs absorption spectrum.

Comparison chart

Action Spectrum Absorption spectrum
1. Action spectrum represents the total rate of photosynthesis at each wavelength of light. 1. Absorption spectrum represents the wavelength of light that is exploited by various kinds of pigments.
2. Single disclosure to full light is required in the case of the action spectrum. 2. The light of various wavelengths is required for the absorption spectrum.
3. It concludes the relationship for the different wavelengths in order to photosynthetic activity. 3. It concludes the relationship between the absorbing competence of pigments along with the quality of light.
4. Blue and red lights are used when photosynthesis occurs in the case of the action spectrum. 4. In the absorption spectrum, chlorophyll absorbs red light and blue light. Besides, carotenoids absorb blue light as well as violet light.
5. By measuring the quantity of carbon dioxide fixation, the rate of photosynthesis is observed. Besides, NADP reduction along with oxygen production is observed in the action spectrum. 5. Spectrophotometer is used for measuring the absorption of different wavelengths of light due to various pigments.

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Key differences between the action spectrum and absorption spectrum

The comparison cannot be completed without knowing the differences. So let’s know the differences between these two.

Action Spectrum

By Daniele Pugliesi (derived by M0tty from original), CC BY-SA 3.0, Link


Absorption Spectrum

absorption spectrum
Graph courtesy of

Laboratory application:

Action spectrum has a vast use in determining the most effective wavelength of light. In a specific chemical reaction, some situations arise which require determining the wavelength of light. Also,  it’s complicated to finish the chemical reaction without using the most effective wavelength. Hence, the action spectrum helps to find the desired wavelength of light through a graphical representation.

On the other hand, an absorption spectrum is an essential tool for analytical chemistry. In some cases, it’s a must to identify the exact substance from the chemical sample. The absorption spectrum helps to do that. Moreover, it’s possible to determine the amount of the desired substance with the help of the absorption spectrum.

Connection difference:

The action spectrum expresses the connection between photosynthetic activity and various wavelengths of light. Hence, it’s not possible to determine the quality of the light using the action spectrum.

In the case of the absorption spectrum, it is possible to express the connection between the quality of light and various pigments absorbing capacities. The different wavelengths of light act differently on various pigments.

Real-time purpose

One of the core purposes of the action spectrum is optimizing plant growth. In addition, the action spectrum helps to find if the metabolism of the plant is appropriate. However, the action spectrum is a part of electromagnetic radiation. It helps the action spectrum to find the total rate of photosynthesis in plants.

On the flip side, an absorption spectrum is a basic tool for absorbance spectroscopy. It’s a kind of molecular spectroscopy process. Hence, molecular spectroscopy is essential for understanding atomic structures. Besides, it’s helpful to find the components in an unknown material effectively.

The absorbance of different colors of light

In the action spectrum, blue light, as well as red light, creates the most effective impact. Let’s clarify with an example. Chlorophyll shows the highest effectiveness when staying in the red and blue region of the light spectrum. If it’s put on the green light region, the effectiveness will be decreased. To redact photosynthesis properly, the action spectrum helps to find the exact color of light. However, there are different wavelengths for different colors and the action spectrum is assistance in that case.

When dealing with the absorption spectrum, it monitors the wavelengths absorbed by the pigment. You should know that the pigments don’t absorb all the light of different wavelengths. If the dealing is with chlorophylls, blue and orange-red light is included here. Or else, blue and violet lights are absorbed by carotenoids in case of the absorption spectrum.

Individual evaluating method

The individual kinds of evaluating processes are needed for determining the action spectrum and absorption spectrum. For the action spectrum, you are required to measure the carbon dioxide fixation. Along with this, oxygen production and NADP reduction can be helpful for the action spectrum.

However, measuring carbon dioxide fixation can be a pick option for evaluating the action spectrum. In that circumstance, the radioactivity in each disk can calculate the total carbon fixed by each leaf.

On the other side, a spectrophotometer is an assist in the absorption spectrum. This apparatus helps to calculate the actual amount of light absorbed by any sample. The absorption spectrum deals with different kinds of pigment.

Therefore, the amount of absorption of light of different wavelengths mustn’t be equal. So, a spectrophotometer brings convenience to find the exact amount of absorbance. However, different ranges of wavelengths of light can be determined depending on the model of this machine.

Key similarities between the action spectrum and absorption spectrum

Along with the differences, knowing similarities is important to compare anything. So let’s have a look at the similarities between these two spectrum.

Graphical representation

Both the action spectrum and absorption spectrum are dependent on graphical representation. This graphical representation is presented according to the wavelengths. However, the graph of an action spectrum shows the rate of photosynthesis. Its rate varies for variations of wavelengths of light.

But, the graph of an absorption spectrum doesn’t deal with the rate of photosynthesis. Instead of this, it deals with absorbance. An absorption spectrum is a kind of graph that shows absorbance for various wavelengths. Whatever their dealing topic is, both of them express their result through a graph.

Use in photosynthesis

The photosynthesis process is necessary for both the action spectrum and absorption spectrum. However, the action spectrum measures the effectiveness of the various wavelengths of light while photosynthesis runs.

While determining the different wavelengths of light using the absorption spectrum, photosynthesis needs to be run. Different pigments of light absorb different lights in the time of photosynthesis. So, both of the spectra require photosynthesis.


Both the action spectrum and absorption spectrum have some dissimilarities. Moreover, we have portrayed some similarities too. Our effort was to uphold these commonalities along with differences in a perceptible form. After reaching the endpoint, we guess you have earned much knowledge about the action spectrum vs absorption spectrum.

This article is an assist for understanding the definition of the two depicted spectrums. Hence, a reader can easily pick the exact spectrum for relevant purposes. For the convenience of the readers, we have created a table describing the key differences. We will be glad if our toil about distinguishing the spectrums brings some help towards you.

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