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A light beam is described by E = 800 sin(ω(t - x/c)). An electron is allowed to move normal to the propagation of light beam with a speed of 3 × 10⁷ m s⁻¹. What is the maximum magnetic force exerted on the electron?

Asked in JEE Main 26th Aug 2nd Shift 2021 · Force on a charge inside the wave

• (1) 1.28 × 10⁻²¹ N
• (2) 12.8 × 10⁻¹⁸ N
• (3) 1.28 × 10⁻¹⁸ N
• (4) 12.8 × 10⁻¹⁷ N

Step-by-step solution

Why the other options are wrong

• (1) 1.28 × 10⁻²¹ N divides by c twice — B₀ = E₀/c² instead of E₀/c
• (3) 1.28 × 10⁻¹⁸ N uses v = 3 × 10⁶ m s⁻¹ in place of the given 3 × 10⁷ m s⁻¹
• (4) 12.8 × 10⁻¹⁷ N is ten times too large — check 800/(3 × 10⁸) = 2.67 × 10⁻⁶ T, not 2.67 × 10⁻⁵ T

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