Explanation

The intensity of an electromagnetic wave is given by the relation

$$ I = \frac{1}{2} c \varepsilon_0 E_0^2 $$

where $E_0$ is the amplitude of the electric field.

Rearranging the formula,

$$ E_0 = \sqrt{\frac{2I}{c\varepsilon_0}} $$

Substitute the given values,

$$ E_0 = \sqrt{\frac{2 \times 4.0 \times 10^{14}}{3 \times 10^8 \times 8.85 \times 10^{-12}}} $$

$$ E_0 = \sqrt{\frac{8.0 \times 10^{14}}{2.655 \times 10^{-3}}} $$

$$ E_0 = \sqrt{3.01 \times 10^{17}} $$

$$ E_0 \approx 5.48 \times 10^8\ \text{V/m} $$

For an electromagnetic wave, the relation between electric and magnetic field amplitudes is

$$ E_0 = c B_0 $$

So,

$$ B_0 = \frac{E_0}{c} $$

$$ B_0 = \frac{5.48 \times 10^8}{3 \times 10^8} $$

$$ B_0 \approx 1.83\ \text{T} $$

Hence, the amplitude of magnetic field is

1.83 T

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