Choose the incorrect statement for the polarisation by reflection.

Options

A. The reflected light and refracted light is at 90" in complete polarization by reflection
B. If `theta_p`, is angle of polarization, then refractive index should always be greater than tan `theta_p`
C. At the angle of polarization, the percentage of the polarized light in the reflected beam is greatest
D. Both (1) & (3)

Answer

To solve the question regarding the incorrect statement for the polarization by reflection, we will analyze the given options based on the principles of wave optics, particularly Brewster's law.

### Step-by-Step Solution:

1. **Understanding Polarization by Reflection**:
- Polarization by reflection occurs when light reflects off a surface at a specific angle known as Brewster's angle (θ_P).
- At this angle, the reflected light is completely polarized perpendicular to the plane of incidence.

2. **Brewster's Law**:
- Brewster's law states that at the angle of polarization, the tangent of the angle of incidence (θ_P) is equal to the refractive index (μ) of the medium divided by the refractive index of the other medium (which is air, μ = 1).
- Mathematically, this is expressed as:
\[ \tan(θ_P) = μ \]

3. **Analyzing the Options**:
- **Option A**: States that the reflected light and the refracted light are at a 90-degree angle in complete polarization by reflection.
- This statement is true, as at Brewster's angle, the reflected and refracted rays are indeed at 90 degrees to each other.

- **Option B**: States that if θ_P is the angle of polarization, then the refractive index should always be greater than θ_P.
- This statement is incorrect. According to Brewster's law, the correct relationship is that the tangent of θ_P equals the refractive index (μ), not that μ must be greater than θ_P.

- **Option C**: States that at the angle of polarization, the percentage of polarized light in the refracted beam is greatest.
- This statement is true. At Brewster's angle, the reflected light is maximally polarized.

4. **Conclusion**:
- The incorrect statement among the options is **Option B**. Therefore, the answer to the question is Option B.