Dispersion of Light in Spectroscopy

Light sources of spectrometers generally emit a broad spectrum of wavelength bands which require to be separated into individual wavelength lines for use by the spectrometer. The component which plays the role of isolation of a single wavelength and directing it to the sample compartment is called the monochromator. The light dispersion element inside the monochromator is either a prism or a diffraction grating. In case of simultaneous detection a polychromator is used which disperses the light into several distinct wavelengths which after passing through the sample compartment reach an array of detectors.

Light Dispersing Elements

Prisms

Triangular Prism Breaks Light Into Spectral Colors

Light Dispersion by Prism

All of you would be familiar with the dispersion of a prism in your school physics laboratories. A single triangular block of glass disperses light into the familiar rainbow colours. Each colour comprises of a range of specific wavelengths. The ability of the prism has been utilized in spectrometers for selecting specific wavelength lines.

Advantages of Prisms

  • Prisms are highly efficient over the complete visible range from about 400 – 1000 nm
  • Transmittance of light is almost around 100%.

Disadvantages of Prisms

  • Dispersion is not uniform throughout the wavelength spectrum and increases towards shorter wavelengths
  • In most instruments two to three prisms are necessary in series to achieve the desired dispersion
  • Glass strongly absorbs in UV region so fused silica or quartz prisms are necessary in UV range
  • Light losses occur at both entrance and exit interfaces of the prism.

Diffraction Gratings

Diffraction Grating

Schematic of Diffraction Grating

A diffraction grating consists of a surface bearing closely spaced grooves etched on it. The separation between the lines is of the order of magnitude of wavelengths of incident light falling upon it. It splits and deflects the incident wavelengths in different directions.

Gratings can be either reflective or transmitive. In the zero order there is no diffraction and the beam is reflected or reflected as per the nature of the grating. On either side there are other orders resulting due to interference pattern of reflected or transmitted beam. Commonly two types of gratings are in use, namely, ruled ratings and holographic gratings

Ruled Gratings

Both transmission and surface reflective gratings come under this category. The dispersion power of the grating is proportional directly to the number of lines/mm.. A grating presents several orders of diffraction. It can be blazed by selection of the groove angle to produce maximum efficiency at that angle

Holographic Gratings

A holographic grating is formed by an interference fringe field of two laser beams whose standing wave pattern is exposed to a polished substrate coated with a photoresist material. Such gratings are normally less efficient than etched gratings as several dispersion orders do dissipate the light energy but sharper diffraction orders result.

Grism

This is a dispersive element formed by combining a transmission grating with a prism. It balances the light deviation caused by the diffraction grating by the reflective effect of the prism. Though it has found favour in certain situations its dispersion is not linear.

Hope the article has helped clear some of your concepts on light diffraction elements. Please do offer your comments.

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.

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