What is “dispersion of white light”? Draw a labelled diagram to illustrate the recombination of the spectrum of white light. Why is it essential that the two prisms used for the purpose should be identical and placed in an inverted position with respect to each other?

Dispersion of White Light

The splitting of white light into its component colours (VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red) when it passes through a glass prism is called dispersion.

Recombination of the Spectrum of White Light

Below is the diagram showing the recombination of the spectrum of white light using two identical prisms, one upright ($P_1$) and one inverted ($P_2$):

``diagram {"xrange": [-16.8, 466.8], "yrange": [-152.4, -127.6], "lines": [{"from": [30, -150], "to": [125, -140], "label": "White light"}, {"from": [125, -140], "to": [175, -130], "label": "Spectrum (Red to Violet)"}, {"from": [175, -130], "to": [275, -150], "label": ""}, {"from": [325, -140], "to": [420, -150], "label": "Recombined White light"}]} ``

Why Identical and Inverted Prisms are Essential:

1. Identical Prisms: The second prism must be identical to the first so that it has the same refracting angle and refractive index. This ensures that the magnitude of dispersion (the degree of bending/separation of different wavelengths) produced by the second prism is exactly equal to that produced by the first prism.
2. Inverted Position: Placing the second prism in an inverted position ensures that the refraction/bending of light rays occurs in the opposite direction. While the first prism disperses the white light by bending different colours through different angles (violet the most, red the least), the inverted second prism bends these separated colours in the opposite direction by an equal amount, bringing them back together to emerge as a single, parallel beam of white light.