An electric bulb is rated as 200W . What will be the peak magnetic...

Intensity of light is $I = \dfrac{Power}{Area} = \dfrac{200 \times \dfrac{3 . 5}{100}}{4\pi{\left(4\right)}^{2}} = 0 . 034W{m}^{- 2}$

Using the relation between magnetic field and intensity, $I = \dfrac{{B}_{0}^{2}c}{2{\mu}_{0}}$

We have, magnetic field as ${B}_{0} = \sqrt{\dfrac{I \times 2{\mu}_{0}}{c}} = \sqrt{\dfrac{0 . 034 \times 2 \times 4\pi \times {10}^{- 7}}{3 \times {10}^{8}}} = 1 . 69 \times {10}^{- 8}T \approx 1 . 71 \times {10}^{- 8}T$