A light wave is incident normally on a glass slab of refractive index… | JEE Main 2019 (12 Jan, Shift 1)

Q31JEE Main 2019 · 12 JAN · SHIFT 1Difficult

Energy Density & Intensity

A light wave is incident normally on a glass slab of refractive index 1.5. If 4 % of light gets reflected and the amplitude of the electric field of the incident light is 30 V/m, then the amplitude of the electric field for the wave propagating in the glass medium will be:

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Answer: (d)

Transmitted intensity $= 96\%$ of incident intensity
In a medium $I = \tfrac{1}{2}\varepsilon v E^2 = \tfrac{1}{2} n \varepsilon_0 c E^2$ (since $\varepsilon = n^2\varepsilon_0$ , $v = c/n$ )
$n E_t^2 = 0.96\,E_0^2 \Rightarrow E_t^2 = \dfrac{0.96 \times 900}{1.5} = 576$
$E_t = 24\ \text{V/m}$ , so the answer is (d).

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