RE-NEET 2026 Physics EM Waves in a dielectric medium MCQ Question with Solution

An electromagnetic wave travelling in a lossless dielectric medium having a dielectric constant, εᵣ = 9, has the electric field, Eₓ = E₀ sin(kz − 2π×10⁶t) Vm⁻¹ where E₀ is the amplitude and k is the wave vector. Among the following options, the incorrect choice is:

A

The wavelength of the electromagnetic wave inside the medium is 300 m

B

The magnetic field is given by the relation Bᵧ = (B₀/v) sin(kz − 2π×10⁶t) where v is the speed of the electromagnetic wave inside the medium

C

The direction of propagation of the electromagnetic wave is along +z

D

The speed of the electromagnetic wave inside the medium is 10⁸ ms⁻¹

Correct Answer

Option A and Option B — both accepted

The examination board accepted both (A) and (B) as correct for this question. Full marks are awarded to every candidate.

Detailed Explanation

Both option (1) and option (2) were accepted by NTA — every candidate is awarded the marks.

✅ Correct Answers: 1 & 2

🔍 Quick Explanation

Given:

\epsilon_r=9,\qquad \mu_r=1

1️⃣ Wavelength inside medium

v=\frac{c}{\sqrt{\epsilon_r}}=\frac c3

Frequency remains unchanged:

\lambda=\frac vf=\frac{\lambda_0}{3}

🟢 Statement 1 — Correct

2️⃣ Magnetic field

For an EM wave:

B=\frac{E}{v}

Since $v=c/\sqrt{\epsilon_r}$ ,

B=\frac{\sqrt{\epsilon_r}E}{c} =\sqrt{\epsilon_r}\,B_0

Thus, for $\epsilon_r=9$ :

B=3B_0

🟢 Statement 2 — Correct (as given in the question)

❌ Statements 3 & 4

They contradict the relations:

v=\frac{c}{\sqrt{\epsilon_r}},\qquad \lambda=\frac vf

📖 NCERT Line

“The speed of electromagnetic waves in a medium is $v=\frac{1}{\sqrt{\mu\epsilon}}$ .”

⚡ QUICK REVISION

Dielectric medium:

\boxed{f=\text{constant}}

\boxed{v=\frac{c}{\sqrt{\epsilon_r}}}

\boxed{\lambda\propto v}

\boxed{B=\frac Ev}

🧠 Medium changes $v,\lambda,B$ , but frequency remains unchanged.

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RE-NEET 2026 Physics EM Waves in a dielectric medium MCQ Question