Physics

Problem

A plane electromagnetic wave travels through glass of refractive index

n = 1.5

with an electric field amplitude of

6\,\text{V m}^{-1}

. Take

c = 3.0 \times 10^{8}\,\text{m s}^{-1}

. Determine the magnetic field amplitude

B_0

inside the glass. a)

2.0 \times 10^{-8}\,\text{T}

4.5 \times 10^{-8}\,\text{T}

3.0 \times 10^{-8}\,\text{T}

1.3 \times 10^{-8}\,\text{T}

Intuitive explanation

The key is recognizing that the relationship between electric and magnetic field amplitudes in an electromagnetic wave depends on the speed of light in that medium, not in vacuum. Since light travels slower in glass (by a factor of $n$ ), the wave impedance changes—the magnetic field must be proportionally larger to carry the same energy. Use the fundamental wave relation $E_0 = v \cdot B_0$ , where $v = c/n$ is the speed in the medium.

Intuitive explanation

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