The refractive index of a glass prism is minimum for

The correct answer is Option (1) → Red

The refractive index of a transparent material depends on the wavelength of light. For glass it decreases as the wavelength increases (Cauchy's relation \( n = A + B/\lambda^{2} \)).

Among the colours of visible light, red has the longest wavelength and violet the shortest. Green and yellow lie between them, in the order violet, green, yellow, red as the wavelength increases.

    \( n_{\mathrm{violet}} \gt n_{\mathrm{green}} \gt n_{\mathrm{yellow}} \gt n_{\mathrm{red}} \)

So the refractive index of glass, and hence the deviation by a prism, is least for red light.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), but it is still in the CUET 2026 syllabus, so do not skip it.

NCERT (Pre-rationalised edition, 2022-23):
Dispersion by a Prism – “… We now know that colour is associated with wavelength of light. In the visible spectrum, red light is at the long wavelength end (~700 nm) while the violet light is at the short wavelength end (~ 400 nm). Dispersion takes place because the refractive index of medium for different wavelengths (colours) is different. For example, the bending of red component of white light is least while it is most for the violet.”