$(a)$ Explain with the help of a labelled diagram how a combination of two prisms can be used so that incident white light emerges out of the second prism as white light.
$(b)$ Name the scientist who first performed this activity.
$(c)$ Explain the term spectrum in brief.

(N/A) prism $P_{1}$ in the path of white light disperses it into various colours. The red rays are deviated the least and the violet rays are deviated the most. The second prism $P_{2}$ is placed in an inverted position relative to $P_{1}$ such that its refracting edge is at the bottom. The second refracting surface of $P_{1}$ and the first refracting surface of $P_{2}$ are parallel. The dispersion produced by $P_{1}$ is neutralised by $P_{2}$. The colours recombine in prism $P_{2}$ and a beam of white light emerges from it.
$(b)$ Isaac Newton.
$(c)$ Spectrum: It is the band of coloured components of light $(VIBGYOR)$ that is obtained when a beam of white light is passed through a glass prism.