Lecture Notes: Wave Theory of Light

Introduction

Shining a Light on a Coin: When you shine light on a coin in mid-air, its shadow will have a bright spot in the middle.
- This counterintuitive observation underpins the wave nature of light.
- Initially surprising to 19th-century physicists.

17th-18th Century: Dominant belief that light is made of particles.
Wave Theory Proposition: Over time, evidence accumulated suggesting light behaves like a wave.
Dual Nature of Light: Today, we understand light as having both particle and wave characteristics.

Christian Huygens: Dutch physicist who posited light as a wave.
Principle Explanation:
- Predict future wave position by analyzing current position.
- Each point on a wave is the source of its own tinier wave (wavelets).
Application Example:
- Wave traveling at 10 cm/s: After 2 seconds, predict wave’s future position using wavelets and tangential curves.

Concept: Waves are reshaped by obstacles they encounter, called diffraction.
Obstacle Interaction: When waves hit an edge, they form wavelets that bend around the edge.
Slit Interaction: Waves passing through a slit curve outward, spreading in a circular pattern.
Particle Comparison: Particles hitting an edge would continue straight, unlike waves which spread.
Bright Spot in Coin's Shadow: Due to diffraction and interference around the edges.

Constructive Interference: When wave crests and troughs align (increase amplitude; bright spots).
Destructive Interference: When wave crests align with troughs (decrease amplitude; dark regions).

Thomas Young (1801): Key experiment demonstrating wave-like behavior of light.
- Narrow beam of sunlight through two slits creates a pattern of bright and dark lines on a screen.
Diffraction Pattern: Resulting from light interference (constructive & destructive).
Path Difference: Difference in distance traveled by waves leading to interference outcomes.

Intensity: Energy transported by light per unit area over time, proportional to amplitude squared.
- Example: Doubling amplitude quadruples brightness.
Frequency & Wavelength: Determine light's visibility and color.
- Higher frequency/shorter wavelength: Blue.
- Lower frequency/longer wavelength: Red.
- White light: Combination of various colors.

Pattern: Series of lines dimming farther from the center.
- Brightest at the center, due to zero path difference.
Path Difference: Light from top vs. bottom of slit determines interference (constructive or destructive).

Coin's Shadow: Bright spot in the center due to diffraction and constructive interference.
- Waves from coin's edge travel equal distances, combining crests and troughs.

Key Learnings:
- Wave theory of light supported by Huygens' Principle, diffraction, and interference.
- Double slit and single slit experiments demonstrate light’s wave properties.

Reference: Produced by Crash Course Physics in association with PBS Digital Studios.