In the previous post we studied the following equation for Beer – Lambert’s law –

We understood that if we decide the intensity of incident light ( o) and then measure the intensity of transmitted light ( t) , we can use this equation to determine concentration(c) of an unknown solution. Let us now proceed to learn how to measure these parameters experimentally and how to use the data obtained for determining concentration of an unknown species.


Spectrophotometry is a method to measure how much light (UV, visible or IR) is absorbed/ transmitted by a solution, by passing light through it. A device used for this purpose is called a spectrophotometer.

Depending on the range of wavelength of the light source, spectrophotometers can be classified as –

  • UV-visible spectrophotometer: light in the ultraviolet range (185 – 400 nm) and visible range (400 – 700 nm) is used
  • IR spectrophotometer: light in the infrared range (700 – 15000 nm i.e 1.5mm ) is shone upon the sample.

A typical block diagram for a spectrophotometer can be shown as follows –

A spectrophotometer is generally made of two devices –

i)Spectrometer – which produces light of a desired wavelength
ii)Photometer – which measures the intensity of transmitted light.

The typical components of a spectrophotometer are –

  1. Light source(LS) – Many different types of light sources are used to produce UV and visible light. However, no one light equipment can meet all the requirements.
  2. Collimator lens (CL) – A collimator lens produces parallel beams of radiation. It thus narrows and aligns the light coming from the light source in a specific direction. How does a collimator work ? Watch the following video – https://youtu.be/–nV2WCwjpM
  3. Filter / Monochromator (F)
    MONO = one / single , CHROMA = light
    As the name suggests , a monochromator helps produce light of a specific wavelength or a set of desired wavelengths. Thus, we get a narrow band of desired wavelength , which then can be shone upon the sample solution.
  4. Slit(S) – This is a narrow opening , which controls the amount of light entering the cuvette.
  5. Cuvette (C)– This is the container that holds the sample solution under study.
  6. Detector (D) – A detector is a device that measures the intensity of transmitted light or absorbance. The most common type of detector in spectrophotometry is a photomultiplier tube (PMT).

After briefly understanding the components of a spectrophotometer , we will study each component in more detail in the coming posts. Till then ,

Be a perpetual student of life and keep learning….

Good day !

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