How Does a Spectrometer Work?

Of all the instruments developed by man, the one that has made exploration of faraway light sources possible and helped us understand the composition of stars, including our Sun, is a spectrometer. This device is used to study emission and absorption spectra of various materials in physics, chemistry, and astronomy.

Also known as a spectrophotometer, spectrograph, or spectroscope, it is an instrument used to analyze the nature of light, that is emitted by various sources. It is used to analyze the properties of light, including its intensity variation, according to wavelength.

There are various types of spectrometers but their basic objective of construction is the analysis of light, emitted by any kind of source.

Light is not just one electromagnetic wavelength but consists of a bunch of electromagnetic waves, of different wavelengths. Just like every person has a unique thumbprint, every element has its signature emission or absorption spectrum. A spectroscope splits light into its component wavelengths, which are recorded.

By having a look at the emission or absorption spectrum, which is like a series of spectroscopic thumbprints of elements, the composition of a light source can be known.

The science of analyzing these spectra and interpreting them is called spectroscopy. It involves the analysis of light, over a range of wavelengths, including visible light. Helium was discovered from analysis of the light spectrum, emitted by the solar chromosphere.

This was revealed by the use of a spectrometer. It continues to play an important role in space exploration. Those fitted on rovers that landed on Mars, helped in analyzing the material composition of the Martian surface.

A ray of light enters a spectrometer, through a collimator slit (which converts the incoming light into a parallel beam) and falls on the diffraction grating. These gratings are surfaces with several thousand lines etched into them, which cause the light to get diffracted.

The readable side of a DVD or a compact disc, for example, acts as a diffraction grating, splitting incident visible light, into its fundamental wavelength components.

The grating causes the light to diffract and split into its component wavelengths. Then, the diffracted light falls on mirrors, which direct it to fall on a CCD (Charged Coupled Device), that reads the intensity of the various light components and converts them into electronic signals.

These signals can be fed into a computer for further analysis. In simple spectrometers, instead of a CCD, the light may just fall on a flat surface, from which the spectrum is analyzed.

The early spectrometers were simpler in their construction, though basic principles were similar to those of modern versions. Instead of a diffraction grating, there used to be a prism in its place, which split the collimated beam of light, into its component wavelengths. These were then made incident on a flat surface.

Using a moving telescope, the angle of diffraction of each wavelength was measured, from which the wavelength was calculated. These types are still used as laboratory equipment in schools and colleges.

The working of a spectrometer may vary according to its construction but the basic principle of operation remains the same. Light enters the slit, gets collimated first (made parallel), and then is passed through a dispersive element (like a prism) or a diffracting element, which splits it into component wavelengths.

Then, the dispersed or diffracted light is reflected by mirrors, onto a plane surface or to a CCD-like photosensor device for analysis.

Instruments like spectrometers are ingeniously built devices, that are useful in applied chemistry, physics and observational astronomy and are responsible for many incredible discoveries.