### Correct Option: A. 30 degrees ### Detailed Explanation: When light strikes a surface, such as a plane mirror, it follows a fundamental principle known as the law of reflection. This law states that the angle of incidence is equal to the angle of reflection. 1. **Understanding the Terms**: - **Angle of Incidence (i)**: This is the angle formed between the incoming light ray and the normal (an imaginary line perpendicular to the surface at the point of incidence). - **Angle of Reflection (r)**: This is the angle formed between the reflected light ray and the normal. 2. **Applying the Law of Reflection**: - According to the law of reflection: \[ \text{Angle of Incidence (i)} = \text{Angle of Reflection (r)} \] - In this problem, the angle of incidence is given as 30 degrees. 3. **Calculating the Angle of Reflection**: - Since the angle of incidence is 30 degrees, we can directly apply the law of reflection: \[ r = i = 30 \text{ degrees} \] - Therefore, the angle of reflection is also 30 degrees. ### Why Other Options Are Incorrect: - **Option B: 45 degrees**: - This option suggests that the angle of reflection is 45 degrees. However, this contradicts the law of reflection, which states that the angle of reflection must equal the angle of incidence. Since the angle of incidence is 30 degrees, this option is incorrect. - **Option C: 60 degrees**: - Similar to option B, this option incorrectly states that the angle of reflection is 60 degrees. Again, this does not adhere to the law of reflection, which requires the angles to be equal. Thus, this option is also incorrect. - **Option D: 90 degrees**: - This option implies that the angle of reflection is 90 degrees. This would mean that the light ray is reflecting straight back along the normal, which is not possible given that the angle of incidence is only 30 degrees. Therefore, this option is incorrect as well. ### Common Pitfalls: - **Confusing Angle of Incidence with Angle of Reflection**: Students sometimes mix up these two angles. Remember, they are always equal in the case of reflection. - **Misunderstanding the Normal Line**: The normal line is crucial for measuring angles. It is always perpendicular to the surface at the point of incidence. - **Ignoring the Law of Reflection**: Always apply the law of reflection when dealing with mirrors; it is a fundamental principle in optics. ### Revision Summary: - The angle of incidence equals the angle of reflection (Law of Reflection). - For a beam striking a mirror at 30 degrees, the angle of reflection is also 30 degrees. - Always measure angles with respect to the normal line, not the surface. - Remember that the law of reflection applies to all types of mirrors and reflective surfaces.