An electromagnetic wave of intensity 50 W m⁻² enters a non-magnetic medium of refractive index n without any loss. Find the ratio of electric-field magnitude before entry to that after entry, followed by the corresponding ratio of magnetic-field magnitudes.
Options

Question

An electromagnetic wave of intensity 50 W m⁻² enters a non-magnetic medium of refractive index n without any loss. Find the ratio of electric-field magnitude before entry to that after entry, followed by the corresponding ratio of magnetic-field magnitudes.

Options

G1 Physics · Accepted Answer

For a non-magnetic dielectric, average intensity is I=(1/2)ncε₀E². Since there is no loss, intensity is unchanged, so n[E(after)]²=[E(before)]². Hence the electric-field magnitude before entry divided by that after entry is √n. Also B=E/v=nE/c. Therefore the magnetic-field magnitude after entry is √n times the value before entry, so the before/after magnetic-field ratio is 1/√n. Thus the required pair is (√n,1/√n). Option B.