Which of the following colours of light undergoes the least deviation while…

When white light enters a prism, each colour bends by a different amount because the prism's refractive index depends on wavelength, a phenomenon called dispersion. For a thin prism, the angle of deviation D is given by D = (n minus 1) times A, so a larger refractive index n produces a larger deviation.

1. Among the visible colours, wavelength increases in the order violet, blue, green, yellow, orange, red (the VIBGYOR sequence), with violet having the shortest wavelength and red the longest.

2. Refractive index decreases as wavelength increases (Cauchy's dispersion relation), so violet light experiences the highest refractive index and red light the lowest among these colours.

3. Since deviation D = (n minus 1) times A rises with n, the colour with the lowest refractive index bends the least. Among the five options, red has the lowest refractive index, so red undergoes the least deviation.

This matches the familiar order of colours in a rainbow, where red appears on the outer, least-bent arc and violet on the inner, most-bent arc, confirming that red deviates least among visible colours passing through a prism.