A laser beam has intensity of $4.0 \times 10^{14} \mathrm{~W} / \mathrm{m}^{2}$. The amplitude of magnetic field associated with beam is ______ T. (Take $\epsilon_{0}=8.85 \times 10^{-12} \mathrm{C}^{2} / \mathrm{Nm}^{2}$ and $\mathrm{c}=3 \times 10^{8} \mathrm{~m} / \mathrm{s}$)

JEE Main · 2026 · 22 Jan 2026 · Shift 2 · EM Waves

A laser beam has intensity of

4.0 \times 1 0^{14} W / m^{2}

. The amplitude of magnetic field associated with beam is ______ T. (Take

ϵ_{0} = 8.85 \times 1 0^{- 12} C^{2} / Nm^{2}

and

c = 3 \times 1 0^{8} m / s

)

Answer: (D)

The intensity of an electromagnetic wave in terms of the electric field amplitude is

I = \frac{1}{2} ε_{0} E_{0}^{2} c

, so

E_{0} = \frac{2 I}{ε _{0} c}

. The amplitudes are related by

E_{0} = c B_{0}

, hence

B_{0} = \frac{E _{0}}{c} = \frac{1}{c} \frac{2 I}{ε _{0} c}

. Substituting

I = 4.0 \times 1 0^{14} W/ m^{2}

ε_{0} = 8.85 \times 1 0^{- 12}

and

c = 3 \times 1 0^{8} m/s

B_{0} = \frac{1}{3 \times 1 0 ^{8}} \frac{2 \times 4.0 \times 1 0 ^{14}}{8.85 \times 1 0 ^{- 12} \times 3 \times 1 0 ^{8}} \approx 1.83 T .

The amplitude of the magnetic field is

1.83 T

JEE Main · 2026 · 22 Jan 2026 · Shift 2 · Single correct option