Lecture on the Law of Reflection

Introduction to the Law of Reflection

• Definition: The law of reflection states that the angle of incidence is equal to the angle of reflection.

Explanation and Example

• Flat Surface Example:
  • Normal Line: Perpendicular to the surface.
  • Light Ray: Strikes the surface at an angle of incidence.
  • Angle of Incidence: The angle between the light ray and the normal line.
  • Law Application: The ray bounces back at the same angle.
  • Angle of Reflection: Also between the normal line and the light ray.
  • Example Values:
    • Angle of Incidence = 30 degrees
    • Angle of Reflection = 30 degrees

Complementary Angles

• The angles formed with the normal line are complementary:
  • Normal line forms a right angle (90 degrees) with the surface.
  • Example: If one angle is 30 degrees, the complement is 60 degrees.

Problem Solving with Mirrors

• Problem Statement: A light ray strikes a mirror at 30 degrees from the horizontal. Calculate the angle (x) where the ray leaves the second mirror.

Steps to Solve

1. Identify Angles:

   • Normal line makes a 90-degree angle with the surface.
   • Angle of incidence (first mirror) is 60 degrees (90 - 30).
   • According to the law, the angle of reflection (first mirror) is also 60 degrees.
   • Complementary angle is 30 degrees.

2. Triangle Angles:

   • Sum of angles in a triangle = 180 degrees.
   • Calculate the missing angle (y):
     • Equation: 30 + 130 + y = 180
     • Solution: y = 20 degrees

3. Second Mirror Angles:

   • Angle of incidence for the second mirror = 70 degrees (90 - y).
   • Angle of reflection for the second mirror = 70 degrees.
   • Calculate angle x:
     • x = 20 degrees (90 - 70)

Conclusion

• Result: The angle at which the reflected ray leaves the second mirror is 20 degrees.
• Reaffirmation: Angle of reflection for the second mirror is 70 degrees, but the angle at which it leaves (x) is 20 degrees.