Huygen's principle of secondary waves | Wave optics | Physics | Khan

Introduction to Light Behavior

• When parallel rays of light are shone through a small hole in cardboard:

  • Expectation (Newton's idea): Light behaves like bullets, either getting blocked or going straight through.

  • Experiment Outcome: Light spreads out, forming a larger spot on a screen.

Light as a Wave - Huygens' Principle

• Huygens' Principle: Fundamental concept that explains light's behavior as a wave.

• Light is considered a wave traveling through an ether medium, forming wavefronts.

  • Wavefront: A surface where the light waves are in phase.

• Important Property:

  • The angle between light rays and wavefronts is always perpendicular.

Evolution of Wavefronts

• Light bulbs emit light in spherical wavefronts.

• Question: How do wavefronts evolve over time in different scenarios?

  • Initial thought: The new wavefront appears simply as a larger version of the old one if no obstacles are present.

  • Complication arises when obstacles (mirrors, lenses, etc.) are present.

Huygens' Principle Explained

• Key Concept: Every point on the wavefront acts as a source of secondary waves (mini ripples).

  • Secondary Waves: New waves produced from each point on the original wavefront after oscillation.

• Construction of a new wavefront:

  • A common tangent (envelope) to all secondary waves represents the new wavefront.

• Visualization:

  • Each point on the wavefront generates its own secondary wave, creating a pattern that can be analyzed.

Applying Huygens' Principle

• Working with any shape of waveforms: It accurately predicts the evolution of wavefronts regardless of initial form.

  • Secondary waves extend indefinitely from every point, forming a new wavefront through their common tangent.

Addressing Common Questions

• Directionality of Waves: Only the forward direction is considered for common tangent as waves do not travel backwards.

• Fundamental queries about the effectiveness of the method:

  • Scientific theories, including Huygens', may not perfectly encapsulate the phenomenon but are useful for explanations.

Applying Huygens' Principle to Diffraction

• Scene: Parallel rays reaching a cardboard with a small hole.

  • The initial wavefront is a plane wave due to no obstacles.

  • Upon passing through the hole:

    • Each point acts as a source for secondary waves.

    • New wavefront forms by drawing a common tangent through these secondary waves.

  • Result: New wavefront is curved at the edges, leading to the spreading of light.

Summary

• Essential takeaways:

  • Huygens' principle explains why light expands when passing through a tiny hole.

  • Light behaves as waves, leading to phenomena such as diffraction that cannot be described through particle theories alone.

Review Questions

• Can you explain Huygens' principle and its application to wavefront construction?

• Can you use Huygens' principle to describe why light spreads when passing through a small hole?