Given below are two statements :
Statement I : A time varying electric field is a source of changing magnetic field and vice-versa. Thus a disturbance in electric or magnetic field creates EM waves.
Statement II : In a material medium, the EM wave travels with speed v = 1/√(μ ₀ ∈ ₀). In the light of the above statements, choose the correct answer from the options given below.

Solution

Physics sir · 4 steps

μ₀ and ε₀ give the speed in vacuum only; a medium uses its own μ and ε.

1. Statement I. Maxwell showed that a changing electric field produces a magnetic field, and a changing magnetic field produces an electric field. Each keeps regenerating the other, so a disturbance in either field travels outward as an EM wave. Statement I is true.
2. Speed in vacuum. The constants μ₀ and ε₀ belong to free space. Put together, they give 1/√(μ₀ ε₀) = c = 3 × 10⁸ m s⁻¹, the speed of light in vacuum.

   v_(medium) = 1/√(μ ε) = 1/√(μᵣ εᵣ μ₀ ε₀) = c/√(μᵣ εᵣ)

3. Statement II. In a material medium you must use μ = μᵣ μ₀ and ε = εᵣ ε₀. For an ordinary medium μᵣ εᵣ > 1, so the wave is slower than c. Writing 1/√(μ₀ ε₀) for a medium is therefore false.
4. So. Statement I is correct and Statement II is false, which is option (c).

The idea to keep. A subscript 0 means vacuum. In any medium the EM wave speed is 1/√(μ ε) = c/n, so always look for μᵣ and εᵣ before trusting a speed formula.

Answer from the NTA final answer key. Solution by PrepWiser.