The speed of light depends upon the nature of the medium through which it passes. As a result, the beam of light is deviated or refracted from its original path as it passes from one medium to another. It is observed that when a ray of white light is passed through a prism, the wave with shorter wavelength bends more than the one with a longer wavelength. Since ordinary white light consists of waves with all the wavelengths in the visible range, a ray of white light is spread out into a series of coloured bands called spectrum. The light of red colour which has longest wavelength is deviated the least while the violet light, which has shortest wavelength is deviated the most. The spectrum of white light, that we can see, ranges from violet at 7.50 × 10¹⁴ Hz to red at 4×10¹⁴ Hz. Such a spectrum is called continuous spectrum.

Continuous because violet merges into blue, blue into green and so on. A similar spectrum is produced when a rainbow forms in the sky. Remember that visible light is just a small portion of the electromagnetic radiation (Fig.2.7). When electromagnetic radiation interacts with matter, atoms and molecules may absorb energy and reach to a higher energy state. With higher energy, these are in an unstable state. For returning to their normal (more stable, lower energy states) energy state, the atoms and molecules emit radiations in various regions of the electromagnetic spectrum.

A. Emission and Absorption Spectra

The spectrum of radiation emitted by a substance that has absorbed energy is called an emission spectrum. Atoms, molecules or ions that have absorbed radiation are said to be “excited”. To produce an emission spectrum, energy is supplied to a sample by heating it or irradiating it and the wavelength (or frequency) of the radiation emitted, as the sample gives up the absorbed energy, is recorded. An absorption spectrum is like the photographic negative of an emission spectrum. A continuum of radiation is passed through a sample which absorbs radiation of certain wavelengths. The missing wavelength which corresponds to the radiation absorbed by the matter, leave dark spaces in the bright continuous spectrum.

The study of emission or absorption spectra is referred to as spectroscopy. The spectrum of the visible light, as discussed above, was continuous as all wavelengths (red to violet) of the visible light are represented in the spectra. The emission spectra of atoms in the gas phase, on the other hand, do not show a continuous spread of wavelength from red to violet, rather they emit light only at specific wavelengths with dark spaces between them. Such spectra are called line spectra or atomic spectra because the emitted radiation is identified by the appearance of bright lines in the spectra.

B. Characteristic emission spectra

When chemical substances are vaporized in a flame they emit light. This light typically covers a range of wavelengths and, when appropriately dispersed, so that different wavelengths travel in different directions, it reveals what is known as the emission spectrum of substance. The spectrum emitted by each of the chemical element is of particular interest and is known as the characteristic emission spectrum of the element. Spectroscopy is the branch of science that is concerned with the study of spectra; it originated in the early nineteenth century and had developed considerably by the end of the 1850s.

A simple experimental arrangement for the observation of an emission spectrum is shown in figure 1.

A spectrometer usually contains a collimator, which produces a parallel beam of light from a slit, a dispersion device (usually a diffraction grating or a triangular glass prism) and a telescope. As the beam of light from the collimator passes through the dispersion device the constituent wavelengths in the beam to travel in different directions, so they appear at different angles when viewed through the telescope,if the dispersion device is a diffraction grating, light of a given wavelength γ will be observed at particular angles θ₁;θ₂ ;θ₃ etc. as given by the grating relation nγ= d sinθ_n

Where n is the order of diffraction and d is the grating apacing  ( i.e. the distance between adjacent slits on the grating).

Line emission spectra are of great interest in the study of electronic structure. Each element has a unique line emission spectrum. The characteristic lines in atomic spectra can be used in chemical analysis to identify unknown atoms in the same way as finger prints are used to identify people. The exact matching of lines of the emission spectrum of the atoms of a known element with the lines from an unknown sample quickly establishes the identity of the latter, German chemist, Robert Bunsen (1811-1899) was one of the first investigators to use line spectra to identify elements. Elements like rubidium (Rb), caesium (Cs) thallium (Tl), indium (In), gallium (Ga) and scandium (Sc) were discovered when their

minerals were analysed by spectroscopic methods. The element helium (He) was discovered in the sun by spectroscopic method.

1.   Line Spectrum of Hydrogen

When an electric discharge is passed through gaseous hydrogen, the H2  molecules dissociate and the energetically excited hydrogen atoms produced emit electromagnetic radiation of discrete frequencies. The hydrogen spectrum consists of several series of lines named after their discoverers. Balmer showed in 1885 on the basis of experimental observations that if spectral lines are expressed in terms of wavenumber (ν ), then the visible lines of the hydrogen spectrum obey the following formula :

A. Atomic emission.

 The light emitted by a sample of excited hydrogen atoms (or any other element) can be passed through a prism and separated into certain discrete wavelengths. Thus an emission spectrum, which is a photographic recording of the separated wavelengths is called as line spectrum. Any sample of reasonable size contains an enormous number of atoms. Although a single atom can be in only one excited state at a time, the collection of atoms contains all possible excited states. The light emitted as these atoms fall to lower energy states is responsible for the spectrum.

1.   Atomic absorption.

When white light is passed through unexcited atomic hydrogen and then through a slit and prism, the transmitted light is lacking in intensity at the same wavelengths as are emitted in (a) the recorded absorption spectrum is also a line spectrum and the photographic negative of the emission spectrum