Overview
This lecture explains the phenomenon of refraction, why light bends when entering a new medium, and the physical mechanism behind the change in light's speed, including the effects on color separation.
What Is Refraction?
- Refraction is the bending of light when it passes from one medium to another due to a change in speed.
- The amount of bending depends on the degree of speed change and the angle of incidence.
Wave Model & Wavefronts
- Light is modeled as an electromagnetic wave with oscillating electric and magnetic fields.
- Wavefronts are lines or surfaces connecting points that are in phase (oscillating together).
- The direction of wave travel is always perpendicular to the wavefront.
Why Changing Speed Bends Light
- When light enters a medium at an angle, one part of the wavefront slows down first, causing the wavefront to bend.
- If light enters perpendicularly (normal incidence), the entire wavefront slows down equally and there is no bending.
Determining the Degree of Bending
- The extent of light bending depends on how much its speed changes and the angle at which it strikes the surface.
- Less speed change or a smaller angle of incidence results in less bending.
Why Light Slows Down in a Medium
- Light interacts with electrons in the medium, causing electrons to oscillate and emit secondary electromagnetic waves.
- These secondary waves interfere with the original light wave, producing a "pullback" effect.
- The continuous pullback causes light to appear slower as it moves through the medium.
Color Separation (Dispersion)
- Different colors (frequencies) of light experience different degrees of pullback, so they travel at different speeds in glass.
- Violet light slows down and bends more than red light, causing color separation in prisms (dispersion) and rainbows.
Key Terms & Definitions
- Refraction — The bending of light as it passes between media of different densities due to speed change.
- Wavefront — A line or surface connecting points of a wave that are in the same phase.
- Interference — The combination of two or more waves resulting in a new wave pattern.
- Constructive Interference — When waves add to make higher peaks and deeper valleys.
- Destructive Interference — When waves partially or fully cancel each other out.
- Dispersion — Separation of light into colors based on differing speeds in a medium.
Action Items / Next Steps
- Review the process of how wavefronts bend during refraction.
- Practice drawing refraction paths for various angles of incidence.
- Prepare for discussion on geometric optics and image formation in upcoming lessons.