The electric field of an electromagnetic wave travelling through a medium is given by E(x, t) = 25 sin (2.0 × 10¹⁵ t - 10⁷ x) n then the refractive index of the medium is ______. (All given measurement are in SI units) (a) 2 (b) 1.2 (c) 1.5 (d) 1.7

Answer: (c). From E = E₀ sin (ω t - kx): ω = 2.0 × 10¹⁵ rad/s, k = 10⁷ m ⁻¹. v = ω/k = 2 × 10⁸ m/s n = c/v = 3 × 10⁸2 × 10⁸ = 1.5 So the answer is (c) 1.5.

The electric field of an electromagnetic wave travelling through a… | JEE Main 2026 (28 Jan, Shift 1)

Q3JEE Main 2026 · 28 JAN · SHIFT 1Medium

Speed of EM Waves in a Medium

The electric field of an electromagnetic wave travelling through a medium is given by $\vec{E}(x, t) = 25 \sin\left(2.0 \times 10^{15} t - 10^{7} x\right) \hat{n}$ then the refractive index of the medium is ______. (All given measurement are in SI units)

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Answer: (c)

From $E = E_0 \sin(\omega t - kx)$ : $\omega = 2.0 \times 10^{15}$ rad/s, $k = 10^{7}$ m $^{-1}$ .
$v = \omega/k = 2 \times 10^{8}$ m/s
$n = c/v = \dfrac{3 \times 10^{8}}{2 \times 10^{8}} = 1.5$
So the answer is (c) 1.5.

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Speed of EM Waves in a Medium question from JEE Main 2026 (28 Jan, Shift 1)

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