Know about the types of  SPECTRUM,” Impure spectrum and Pure spectrum ”


 “The band of colors lying side by side is called spectrum”. The spectrum may be pure spectrum or impure spectrum.


(i) Impure spectrum:

Impure spectrum is a spectrum in which the constituent colors overlap each other. Light, after passing through v a narrow slit ‘S, strikes a refracting face AB of the prism ABC (Fig.1). Let rays 1 and 2 are the extreme rays of a divergent beam. Both these rays subtend different angles of incidence’s. Since deviation produced by a prism depends upon the angle of incidence, emergent rays of a particular color originating from Fig.1 Production of an impure spectrum by a prism these rays will suffer different deviations. The result is that in central portion of the spectrum, different colors overlap each other producing an impure spectrum. Only the corner colors are pure.

impure spectrum

Fig.1: production of an impure spectrum by a prism                                       

(ii)Pure spectrum:

 Pure spectrum is a spectrum in which all the constituent colors occupy different and distinct positions. If, somehow or the other, rays 1 and 2 in Fig.1 are made to be incident on face AB at same angles of incidence, the rays of a particular color in the emergent beam will be parallel to themselves. A set of parallel rays can be focused on the focal plane lens, thus, avoiding overlapping of colors. The experimental set up for obtaining a pure spectrum is shown in Fig.2 Light after passing through a narrow of a convex slits rendered parallel by means of a convex lens Ly. This will happen when distance between slit “S” and the lens ‘L1 is equal to the focal length of the lens. This parallel am is incident on face AB of a prism ABC. In the emergent beam rays of a particular color are parallel to themselves. Beams of different colors are inclined differently to a convex lens L2’ which brings different colors to focus at different points on a screen be XY placed at its focal plane.

pure spectrum

Fig.2: production of a pure spectrum by a prism


Following conditions have to be maintained in order to obtain a pure spectrum.

(i) The slit ‘S’ should be narrow. A broader slit is equivalent to a number of slits placed side by side. It is possible that pure spectra obtained due to -different parts of a broader slit may overlap each other. To avoid this, slit is made as narrow as possible.

(ii) Use of convex lens L1. Lens ‘L1′ is used to render the incident rays parallel so that all the rays have same angle of incidence.

(iii)Prism to be placed in minimum deviation position. Rate of increase of deviation a different for different colors. So chances of overlapping of colors are minimum if the prism is placed in minimum deviation position.

(iv) Use of convex lens L2. Since beams of different colors are inclined differently to convex lens L2 they can be focused at different points by using the lens.

Also read:


It is an instrument used to obtain a pure spectrum
in laboratory

Construction: The fundamental arrangement of various components of spectrometer is shown in Fig.2 Construction of different parts of the spectrometer is as follows:

(i) Collimator. It is an arrangement of two tubes, one fitted with an adjustable slit “S’ and the other with convex   lens L1 (Fig.3). One tube can be slided into the other with rack and pinion arrangement R1 thereby changing the distance between S and L1. Collimator is mounted on a fixed pillar

collimator, pure and impure spectrum

Fig3: collimator

(ii) Prism table. It consists of combination of two circular discs D1 and D2 held parallel to each other by means of three parallel screws P, Q and R [Fig.4j Lower disc D2 is welded to a rod R which can be adjusted in an upright, thereby changing height of prism table. Prism table can be given a minor rotation by means of tangent screw

pure and impure spectrum

Fig4: prism table

(iii) Telescope. It also consists of a combination of two tubes. In which one can be slided in to other by means of a rack and pinion arrangement R2 [Fig.5] It can also be given minor rotation with the help of a tangent screw.

pure and impure spectrum

Fig5: telescope

(iv) Vernier system of scale. It consists of a main scale circular in form, attached with two systems V1, and V2, 180°apart. The scale is attached to telescope and rotates along with it while the vernier systems are attached to the prism table. A combined arrangement of various parts of spectrometer is shown in Fig.6

pure and impure spectrum

fig6: spectrometer


Setting. Following steps are observed for setting a spectrometer:

(i) Level the spectrometer by means of the leveling screw provided at its base,

(ii) Level the prism table by means of screw P, Q and R.

(iii) Set the telescope for parallel rays by adjusting ‘R2.

(iv) Place the source in front of S. Bring the telescope in line with the collimator. Adjust R1 so that the slit is sharp in focus.


(v) Adjust the width of the slit to have the slit as narrow as possible.

(vi) Place the prism on the prism table so that light is incident on one of its faces.

(vi) Look through the other face. A spectrum is seen. Bring the telescope along line of view. A clear spectrum can be seen through the telescope.

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