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Light is incident normally on a completely absorbing surface with an energy flux of 25 W cm⁻². If the surface has an area of 25 cm², the momentum transferred to the surface in 40 min time duration will be

Asked in JEE Main 10th April 2nd Shift 2019 · Momentum delivered: absorbed or reflected

• (1) 1.4 × 10⁻⁶ N s
• (2) 5.0 × 10⁻³ N s
• (3) 6.3 × 10⁻⁴ N s
• (4) 3.5 × 10⁻⁶ N s

Step-by-step solution

Why the other options are wrong

• (1) 1.4 × 10⁻⁶ N s is 625 × (40/60) / (3 × 10⁸) — you left the exposure in hours instead of converting 40 min to 2400 s
• (3) comes from needs U ≈ 1.9 × 10⁵ J, not 625 W × 2400 s = 1.5 × 10⁶ J
• (4) 3.5 × 10⁻⁶ N s needs U ≈ 10³ J — under two seconds of exposure, not 40 min

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