JEE Main · 2026 · 23 Jan 2026 · Shift 2 · EM Waves

The ratio of speeds of electromagnetic waves in vacuum and a medium, having dielectric constant

k = 3

and permeability of

μ = 2 μ_{0}

, is (

μ_{0} =

permeability of vacuum)

(A) $36 : 1$
(B) $3 : 2$
(C) $6 : 1$
(D) $6 : 1$ ✓ Correct

Answer: (D)

Solution

The speed of an electromagnetic wave in a medium is

v = \frac{1}{μ ε} = \frac{c}{μ _{r} ε _{r}}

, so its refractive index is

n = μ_{r} ε_{r}

. With

ε_{r} = k = 3

and

μ_{r} = 2

n = 3 \times 2 = 6

. The ratio of the speed in vacuum to that in the medium is

\frac{c}{v} = n = 6

, that is

6 : 1

JEE Main · 2026 · 23 Jan 2026 · Shift 2 · Single correct option

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