Physics [5 Mark Question Answer]

The three conditions necessary to obtain a pure spectrum on a screen are:
(i) A narrow parallel beam of light should be made incident on the prism.
(ii) The prism should be placed in the position of minimum deviation.
(iii) The emergent parallel beam should be focussed on the screen.
These conditions are fulfilled in the spectrometer.

If a blackened bulb thermometer is moved from the violet end towards the red end, first a steady rise in temperature is observed, but as the thermometer goes beyond the red end, there is a rapid rise in temperature. This shows the existence of some kind of radiation producing the heating effect beyond the red end of the spectrum.
If the radiations from the red end to the violet end are made to fall on the silver chloride solution, it almost remains unaffected beyond the violet end of the spectrum.

In a prism, the refraction of light takes place at the two slant surfaces. The dispersion of white light occurs at the first surface of the prism where its constituent colours have deviated through different angles. At the second surface, these split colours suffer the only refraction and they get further separated. But in a rectangular glass block, the refraction of light takes place at the two parallel surfaces. At the first surface, although the white light splits into its constituent colours on refraction, these split colours on suffering refraction at the second surface emerge out in form of a parallel beam, which gives an impression of white light.