### Correct Option: B. 30 degrees ### Detailed Explanation: 1. **Understanding the Law of Reflection**: The law of reflection states that when a light ray strikes a reflective surface (like a mirror), the angle of incidence is equal to the angle of reflection. This law can be expressed mathematically as: \[ \theta_i = \theta_r \] where: - \(\theta_i\) is the angle of incidence (the angle between the incoming light ray and the normal to the surface), - \(\theta_r\) is the angle of reflection (the angle between the reflected light ray and the normal). 2. **Identifying the Angles**: In this problem, the beam of light strikes the mirror at an angle of 30 degrees to the normal. This means: \[ \theta_i = 30^\circ \] 3. **Applying the Law of Reflection**: According to the law of reflection, since the angle of incidence is 30 degrees, the angle of reflection must also be: \[ \theta_r = 30^\circ \] 4. **Conclusion**: Therefore, the angle of reflection when the beam of light strikes the flat mirror at an angle of 30 degrees to the normal is also 30 degrees. ### Explanation of Other Options: - **Option A: 15 degrees**: This option is incorrect because it does not follow the law of reflection. The angle of reflection must equal the angle of incidence, which is 30 degrees, not half of it. - **Option C: 60 degrees**: This option is also incorrect. It suggests that the angle of reflection is greater than the angle of incidence, which contradicts the law of reflection. - **Option D: 30 degrees**: This option is actually correct, but it is listed twice (B and D). Therefore, while it is correct, it is redundant to have it as a separate option. ### Common Pitfalls: - **Misunderstanding the Normal**: Students sometimes confuse the normal line (an imaginary line perpendicular to the surface at the point of incidence) with the surface of the mirror itself. Always remember that angles are measured from the normal, not the surface. - **Confusing Angles**: Ensure you do not confuse the angle of incidence with the angle of reflection. They are equal, but they are measured from the normal in opposite directions. ### Revision Summary: - The angle of incidence equals the angle of reflection (Law of Reflection). - For a beam striking a mirror at 30 degrees to the normal, the angle of reflection is also 30 degrees. - Always measure angles from the normal line, not the surface. - Be cautious of redundant options in multiple-choice questions.