Read the following statements carefully and identify the correct statements.
(A) Most interference and diffraction effects exist even for longitudinal waves like sound in air.
(B) Optical fibers work on the phenomenon of total internal reflection.
(C) Optical density and mass density are the same.
(D) Polarisation phenomena are specific to transverse waves like light waves.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (B) and (D) only
(A) Most interference and diffraction effects exist even for longitudinal waves like sound in air. — Correct. Interference and diffraction depend only on superposition and on the wave nature, so sound shows them too.
(B) Optical fibers work on the phenomenon of total internal reflection. — Correct. Light is trapped in the core by repeated total internal reflection at the core-cladding boundary.
(C) Optical density and mass density are the same. — Incorrect. Optical density refers to the refractive index of a medium. A medium can be optically denser yet have a lower mass density; for example, turpentine has a higher refractive index than water but a lower mass density.
(D) Polarisation phenomena are specific to transverse waves like light waves. — Correct. Polarisation needs a direction of vibration perpendicular to the direction of travel, which longitudinal waves such as sound do not have.
NCERT (2026-27):
Points to Ponder – “… 4. Most interference and diffraction effects exist even for longitudinal waves like sound in air. But polarisation phenomena are special to transverse waves like light waves.”
Total Internal Reflection in Nature and Its Technological Applications – “… Optical fibres too make use of the phenomenon of total internal reflection.”
Refraction – “… Note: Optical density should not be confused with mass density, which is mass per unit volume. It is possible that mass density of an optically denser medium may be less than that of an optically rarer medium (optical density is the ratio of the speed of light in two media).”