A parallel beam of light travelling in water is incident obliquely on a glass surface. After refraction its width ………………….

Options

A. decreases
B. increases
C. remains the same
D. becomes zero

Answer

To solve the problem of how the width of a parallel beam of light changes when it travels from water to glass, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Mediums**:
- Water is a rarer medium compared to glass, which is denser. This means that the refractive index of glass is higher than that of water.

2. **Incident Beam of Light**:
- A parallel beam of light is incident obliquely at the interface between water and glass.

3. **Drawing the Diagram**:
- Draw the interface between water and glass. Mark the incident beam of light as it approaches the interface. Draw the normal line (a perpendicular line to the surface at the point of incidence).

4. **Identifying Angles**:
- Let the angle of incidence be \( I \) (the angle between the incident ray and the normal). The angle of refraction \( R \) (the angle between the refracted ray and the normal) will be less than \( I \) because light bends towards the normal when moving from a rarer to a denser medium.

5. **Using Snell's Law**:
- According to Snell's Law, \( n_1 \sin I = n_2 \sin R \), where \( n_1 \) is the refractive index of water and \( n_2 \) is the refractive index of glass. Since \( n_2 > n_1 \), it follows that \( R < I \).

6. **Analyzing the Width of the Beam**:
- The width of the beam before refraction is represented by the distance between two rays (let's call it \( PQ \)). After refraction, the distance between the refracted rays (let's call it \( RS \)) will be affected by the angles \( I \) and \( R \).

7. **Calculating the Width Change**:
- The relationship between the widths can be expressed using the cosine of the angles:
\[ \frac{PQ}{RS} = \frac{\cos I}{\cos R} \]
- Since \( R < I \), it follows that \( \cos R > \cos I \). Therefore, \( \frac{\cos I}{\cos R} < 1 \), which implies that \( PQ < RS \).

8. **Conclusion**:
- The width of the beam after refraction (RS) is greater than the width before refraction (PQ). Thus, the width of the beam increases when it travels from water to glass.

### Final Answer:
The width of the beam of light after refraction increases.