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An object is placed in a medium of refractive index 3. An electromagnetic wave of intensity 6 × 10⁸ W/m² falls normally on the object and it is absorbed completely. The radiation pressure on the object would be (speed of light in free space = 3 × 10⁸ m/s)

Asked in JEE Main 27th Jan 2nd Shift 2024 · Force and radiation pressure

• (1) 18 Nm⁻²
• (2) 6 Nm⁻²
• (3) 2 Nm⁻²
• (4) 36 Nm⁻²

Step-by-step solution

Why the other options are wrong

• (1) 18 N m⁻² applies the refractive index twice, n²I/c = 9 × 2
• (3) 2 N m⁻² is I/c — you used the free-space speed; inside the medium the wave moves at c/n, so the pressure is n times larger
• (4) 36 N m⁻² doubles the already-wrong n²I/c for reflection; the object absorbs completely, so no factor 2 belongs here

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