Overview
This lecture explains the concept of total internal reflection, focusing on the importance of the critical angle, and demonstrates the process using a prism example.
Understanding Total Internal Reflection
- Total internal reflection occurs when light moves from a more dense to a less dense medium (e.g., glass to air).
- The critical angle is the angle of incidence that produces a refracted angle of 90 degrees.
- If the angle of incidence exceeds the critical angle, no refraction occurs; instead, all light is reflected inside the denser medium.
- Total internal reflection only happens when the angle of incidence is greater than the critical angle.
Prism Example: Step-by-Step
- A ray of light enters a glass prism and moves toward a surface at a 45-degree angle of incidence.
- The critical angle for glass is 42 degrees; the incident angle (45 degrees) is greater than the critical angle.
- At the first surface, total internal reflection occurs; the angle of incidence equals the angle of reflection (both 45 degrees).
- The ray then hits another surface at a 45-degree angle, causing another total internal reflection.
- The light ray exits the prism after reflecting twice, following the normal, with no refraction as it leaves.
Key Terms & Definitions
- Total Internal Reflection — Complete reflection of light within a denser medium when the incidence angle exceeds the critical angle.
- Critical Angle — The minimum angle of incidence at which total internal reflection occurs, causing the refracted ray to travel along the boundary.
- Angle of Incidence — The angle between the incoming ray and the normal (perpendicular) to the surface.
- Normal Line — A line perpendicular to the surface where the ray hits.
Action Items / Next Steps
- Review the definition and calculation of the critical angle from previous material.
- Practice drawing normal lines and identifying angles of incidence and reflection in prism problems.