Question

An object is placed in a medium of refractive index 3. An electromagnetic wave of intensity \(6 \times 10^8 \, \text{W/m}^2\) falls normally on the object and it is absorbed completely. The radiation pressure on the object would be (speed of light in free space = \(3 \times 10^8\) m/s) :

• A. \(36 \, \text{Nm}^{-2}\)
• B. \(18 \, \text{Nm}^{-2}\)
• C. \(6 \, \text{Nm}^{-2}\)
• D. \(2 \, \text{Nm}^{-2}\)

Answer 1

The Correct Option is C

The radiation pressure \( P \) is given by:

\(P = \frac{I \cdot \mu}{c},\)

where:
- \( I \) is the intensity of the radiation,
- \( \mu \) is the absorption coefficient (fraction of radiation absorbed, here \( \mu = 1 \) for total absorption),
- \( c \) is the speed of light in a vacuum.

Substitute the given values:
- \( I = 6 \times 10^8 \, \text{W/m}^2 \),
- \( \mu = 3 \),
- \( c = 3 \times 10^8 \, \text{m/s} \).

Calculate \( P \):

\(P = \frac{I \cdot \mu}{c} = \frac{6 \times 10^8 \cdot 3}{3 \times 10^8}.\)

Simplify:

\(P = 6 \, \text{N/m}^2.\)

The Correct answer is: \(6 \, \text{Nm}^{-2}\)