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Thermodynamics

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Spectrum

A beam of light that illuminates the slit of a spectrometer is rendered parallel by the collimating lens of the spectrometer. This parallel beam of light is refracted through a prism and the refracted beam is brought to focus by the objective of the telescope. The image of the slit is viewed with the help of an eyepiece. Optical instruments that are used primarily to study the wavelengths of light present in a given beam of light are called spectrometers.

If the beam of light that illuminates the slit of the spectrometer contains more than one wavelength, then the light waves of different lengths are deviated to different extents by the prism. The image seen in the field of view of the telescope consists of a number of coloured images of the slit. Such an image is called a spectrum. If, on the other hand, a diffraction grating is used in place of a prism, then for any particular order, the diffracting angles corresponding to the direction of the principal maxima are different for different wavelengths of light and hence a spectrum can be obtained in the field of view of the telescope. The observer notices a number of images of the slit side by side and each image of the slit corresponds to a particular wavelength. If the slit is illuminated with light from a sodium vapour lamp, two images of the slit are observed in the yellow region of the spectrum and these are called the D₁ and D₂ emission lines of sodium. Thus, sodium vapour emits radiations of two different wavelengths (5896 Å and 5890 Å) in the visible region.

Similarly, introduction of other metallic salts into a Bunsen flame, result in the production of definite flame colourations and the wavelength of the light emitted by these salts can be studied using a spectrometer. Spectroscopic studies help in the detection of even minute quantities of metallic substances, presents in a given mixture. Caesium, rubidium, thallium etc, were discovered this way.