Identify the incorrect statements with reference to dispersion of white light.
(A) Red light gets most deviated while passing through the prism.
(B) Thick lenses show chromatic aberrations due to dispersion.
(C) Refractive index of medium for different colours is different.
(D) Red light travels faster than violet light in the prism.
(E) White light, once dispersed, cannot be recombined.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (E) only
Dispersion happens because the refractive index of the glass is different for different colours: it is smallest for red and largest for violet. A statement marked Incorrect below is one that we are looking for.
(A) Red light gets most deviated while passing through the prism. — Incorrect. Violet has the largest refractive index, so it is deviated the most; red is deviated the least.
(B) Thick lenses show chromatic aberrations due to dispersion. — Correct. The focal length depends on the refractive index, so different colours focus at different points and the image is coloured.
(C) Refractive index of medium for different colours is different. — Correct. This is exactly what causes dispersion.
(D) Red light travels faster than violet light in the prism. — Correct. \( v = c/n \) and red has the smaller n, so it travels faster in glass.
(E) White light, once dispersed, cannot be recombined. — Incorrect. A second prism placed in the inverted position recombines the dispersed colours into white 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 – “… The red light bends the least, while the violet light bends the most (Fig. 9.25).”
Dispersion by a Prism – “… The explanation was clear— the first prism splits the white light into its component colours, while the inverted prism recombines them to give white light.”
Dispersion by a Prism – “… Dispersion takes place because the refractive index of medium for different wavelengths (colours) is different. … Equivalently, red light travels faster than violet light in a glass prism.”
Dispersion by a Prism – “… Thick lenses could be assumed as made of many prisms, therefore, thick lenses show chromatic aberration due to dispersion of light.”