Which of the following statements are correct?
A. Dispersion is the splitting of light into its constituent colours.
B. The unit for power of a lens is \( \mathrm{m^{-1}} \).
C. Optical fibres consist of glass fibres coated with a thin layer of material of higher refractive index.
D. Cassegrain telescope has the advantages of a large focal length in a short telescope.
E. Glass is a dispersive medium.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A, B, D and E Only
(A) Dispersion is the splitting of light into its constituent colours. — Correct. White light passing through a prism spreads into a spectrum, because each colour is deviated by a different amount.
(B) The unit for power of a lens is \( \mathrm{m^{-1}} \). — Correct. Power is \( P = 1/f \), so with \( f \) in metres the unit is \( \mathrm{m^{-1}} \), called the dioptre (D).
(C) Optical fibres consist of glass fibres coated with a thin layer of material of higher refractive index. — Incorrect. Light is guided by total internal reflection, which happens only when light goes from a denser to a rarer medium. So the cladding must have a lower refractive index than the core.
(D) Cassegrain telescope has the advantages of a large focal length in a short telescope. — Correct. A convex secondary mirror sends the light back through a hole in the concave primary, so the path is folded and a large focal length fits in a short tube.
(E) Glass is a dispersive medium. — Correct. The refractive index of glass depends on the wavelength, so different colours travel at different speeds in it.
Note: Statements (A), (B), (C) and (D) of this question are from the latest textbook (2026-27). Statement (E), that glass is a dispersive medium, is printed only in the pre-rationalised textbook (2022-23). It was removed from the latest book, but dispersion of light through a prism is still in the CUET 2026 syllabus, so do not skip it.
NCERT (2026-27):
Summary – “… 7. Dispersion is the splitting of light into its constituent colour.”
Power of a Lens – “… The SI unit for power of a lens is dioptre (D): 1D = 1m–1.”
Summary – “… Optical fibres consist of glass fibres coated with a thin layer of material of lower refractive index.”
Telescope – “… This is known as a Cassegrain telescope, after its inventor. It has the advantages of a large focal length in a short telescope.”
NCERT (Pre-rationalised edition, 2022-23):
Dispersion by a Prism – “… On the other hand, glass is a dispersive medium.”