Visible Light Spectroscopy Theory

23 Jan
2022

The basis of how visible light spectroscopy works and why it is so useful.

What is spectroscopy in a nutshell?

Spectroscopy is the measurement of radiation intensity as a function of wavelength. There are many different kinds of spectroscopy! This tool is going to focus on visible light spectroscopy

What is visible light spectroscopy?

Coloured light is very often a mixture of different colours. A spectrometer splits the constituent colours of light apart (just like a triangular prism) and can then measure the intensity and strength of each colour. 

What comes out of a visible light spectrometer is called a spectrum. The horizontal axis is the wavelength of light. The vertical axis is the intensity at that wavelength. 

Theory

To understand what spectroscopy is, we need to answer a couple of questions: 

  1. What is light? 

Light can be viewed as electromagnetic radiation, waves on the electromagnetic spectrum, including radio waves, microwaves and infrared. Looking at a wave, the wavelength and frequency determine the nature of the light. Long-wavelength means short frequency and vice versa. 

Electromagnetic radiation in visible light contains quanta (tiny packets of energy called photons). Higher frequency light carries more energy than lower frequency light. 

  1. How does light interact with molecules? 

A molecule is a group of atoms bonded together in some way (topic for another video). Quantum theory says that the energies of things are quantized. This means that molecules and atoms have discrete step-like energy states, and the molecule can only be in one of these states and not ‘in-between’ states. 

When molecules are exposed to the light of the right wavelength, they can be excited, leading to a change in bonding chemistry in the molecule and attaining an excited state. This excitation results in some light being absorbed. 

Light is not the only means of getting molecules excited. They can also be excited by other factors such as heat. When molecules relax from an excited state, they emit the energy associated with the relaxation, which can be given off as light or heat. 

This is why the spectra of heated up molecules often consist of a series of sharp spectral lines, each due to light produced by a specific relaxation in the molecule between quantum states. 

Why is visible light spectroscopy absorption useful?

By examining the spectrum of light before and after it passes through a sample, the difference in the spectrum can be identified through examining spectra. 

Like this, we can get an idea of what absorptions are happening in the sample. 

Qualitative

Many molecules have particular absorption patterns associated with them, allowing us to see if they are in the sample.

Quantitative

The amount of light absorbed at a specific wavelength where absorption occurs also can tell us how much of a compound is in the sample, i.e. its concentration. 

If there is more of a compound in a sample and it absorbs the light shone at it, the intensity will be lower the higher the concentration. 

Fluorescence Spectroscopy

Another technique in spectroscopy is to excite a sample and then measure the emitted light due to the excited molecules relaxing. This works for fluorescent molecules and often is used for analysis of oils. 

Example use-cases

Visible light absorption spectroscopy is used in a wide variety of ways, including: 

  • Identifying adulteration of olive oil with peanut oil. 
  • Identifying pesticides in fruit juice
  • Finding out the concentration of polyphenols in wine
  • Determining the amount of microplastic in water

And many more!

Typical workflow

This tool does not look at the practical use of a spectrometer but rather how you work with one. Check out our tool on building a spectrometer linked below. 

A typical workflow for spectroscopy is the following: 

  • Figure out what you want to test and find out if it absorbs visible light. Something that is coloured, e.g. a red pigment in red wine, absorbs light, but this is not a rule! Sometimes, a colourless substance can change the colour of a coloured one. 
  • Set up a spectrometer and run a blank spectrum (record a light spectrum through a container containing clean (ideally distilled) water. 
  • Take a spectrum of water +  a small amount of the thing you want to test for. 
  • Look at the differences in the spectrum and see whether absorption has occurred. 

If the molecule you wish to test for is absorbing visible light, the following steps would be to identify at what wavelength it does this and the intensity at different concentrations. 

Wilsonee, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

The Beer-Lambert Law

This law relates the concentration of a substance in solution to the intensity of absorbance of light at a wavelength where absorbance occurs. Note that everything such as light source, container, distances, temperature, and surround light should be kept constant to compare measurements. Additionally, the law assumes all measurements have been done at the same wavelength

The law states that the absorbance (the log (base 10) of the intensity of a blank sample light to that containing analyte) is proportional to the concentration of the absorbing analyte in the sample. 

To use the law, a calibration curve must be set up. The absorbance at a specific wavelength is calculated for different analyte concentrations and a blank and a plot constructed. 

When a sample of unknown concentration is measured, the absorption at that wavelength can then be related to the concentration of analyte in the sample. 

See the attached example document for an application.

The Beer-Lambert Law

Resources

Read more about spectroscopy

Spectroscopy simulator

Beer-Lambert Law simulator

Example spreadsheet of a Beer-Lambert Law application


Have we got you excited about spectroscopy? Check out our tool on building one to start analysing your one samples!

References: 

Bunker, P. R., & Jensen, P. (1979). Molecular Symmetry and Spectroscopy — Volumes Publishing. NRC Research Press.

SpectralWorkbench. (n.d.). Retrieved June 27, 2021, from https://spectralworkbench.org/


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