A short pulse of white light is incident from air to a glass slab at normal incidence, after travelling through the slab, the first colour to emerge is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Red
All colours travel the same thickness of glass, so the colour that travels fastest in glass comes out first. The speed in glass is \( v = c/n \), and by dispersion the refractive index of glass is least for red and greatest for violet.
\( n_{\mathrm{red}} \lt n_{\mathrm{green}} \lt n_{\mathrm{blue}} \lt n_{\mathrm{violet}} \)
\( v_{\mathrm{red}} \gt v_{\mathrm{green}} \gt v_{\mathrm{blue}} \gt v_{\mathrm{violet}} \)
So red light emerges first, and violet last.
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 – “… 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. Equivalently, red light travels faster than violet light in a glass prism.”