A person allowed a narrow beam of white light from the sun to enter a dark room through a small aperture and…
A person allowed a narrow beam of white light from the sun to enter a dark room through a small aperture and placed a glass prism in its path in such a manner that the beam falls on the face AB of the prism as shown in the figure.
A screen S is placed on the other side of the prism, facing AC. On turning the prism slowly, a beautiful band of colours is obtained on the screen. It is the spectrum of sunlight.
(a) Name the phenomenon due to which a prism splits the incident white light into a band of colours. [1]
(b) State the reason of getting a band of seven colours in the above case. [1]
(c) (i) Explain with the help of a labelled ray diagram, an experimental arrangement to show the recombination of the spectrum of white light. [2]
Solution
(a) Dispersion of light
(b) Different colours of light bend through different angles with respect to the incident ray as they pass through a prism.
(c) (i) Two identical prisms are placed in inverted position with respect to each other as shown. When spectrum produced by prism A is passed through the prism B, a beam of white light emerges from the other side of the prism B.
[diagram: two prisms arranged base-to-base, white light entering first prism, dispersed R V colours between prisms, white light emerging from second prism onto a screen]
(award full marks even if only labelled ray diagram is given)
