Which of the following statements are true?
(A) The speed of light in vacuum depends on the direction of its propagation.
(B) The speed of light in a medium is independent of the wavelength of light.
(C) The speed of light is independent of the motion of its source.
(D) The speed of light in a medium is independent of its intensity.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C) and (D) only
In vacuum, light travels at the universal speed \( c = 1/\sqrt{\mu_0\varepsilon_0} \). In a medium its speed is \( v = c/n \), where the refractive index n depends on the medium and on the wavelength.
(A) The speed of light in vacuum depends on the direction of its propagation. — Incorrect. Vacuum is isotropic: c is the same in every direction.
(B) The speed of light in a medium is independent of the wavelength of light. — Incorrect. The refractive index of a medium depends on wavelength, so different colours travel at different speeds; this is dispersion. In glass, violet light travels slower than red light.
(C) The speed of light is independent of the motion of its source. — Correct. The speed of light does not depend on the motion of the source (or of the observer); c is fixed by \( \mu_0 \) and \( \varepsilon_0 \) alone.
(D) The speed of light in a medium is independent of its intensity. — Correct. Intensity changes only the amplitude of the wave; the speed is fixed by the medium and the wavelength.