A laser beam has intensity of 4.0 × 10¹⁴ W/m². The amplitude of… | JEE Main 2026 (22 Jan, Shift 2)

Q2JEE Main 2026 · 22 JAN · SHIFT 2Medium

Energy Density & Intensity

A laser beam has intensity of $4.0 \times 10^{14}\text{ W/m}^2$ . The amplitude of magnetic field associated with beam is ______ T.

(Take $\epsilon_o = 8.85 \times 10^{-12}\text{ C}^2/\text{N m}^2$ and $c = 3 \times 10^{8}\text{ m/s}$ )

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Answer: (a)

$I = \tfrac{1}{2}\varepsilon_0 c E_0^2$ and $E_0 = cB_0 \Rightarrow I = \tfrac{1}{2}\varepsilon_0 c^3 B_0^2$
$B_0^2 = \dfrac{2I}{\varepsilon_0 c^3} = \dfrac{8 \times 10^{14}}{8.85 \times 10^{-12} \times 2.7 \times 10^{25}} = 3.35$
$B_0 \approx 1.83$ T
So the answer is (a) 1.83.

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