Light source
Circular obstacle
Wave intensity
Center intensity: 0%
Screen Intensity Profile
← Edge | Center | Edge →
Path length diagram: All edge paths to center are equal
40 px
12 px
200 px

The Paradox

Place a circular obstacle in front of a light source. Behind it, you'd expect a perfectly dark shadow.

Instead, there's a bright spot right at the CENTER of the shadow! This seems impossible—how can light appear where it's supposed to be blocked?

The 1818 Debate

Siméon Poisson, a supporter of the particle theory of light, thought he had found a fatal flaw in Fresnel's wave theory.

Poisson's "proof": "Your wave theory absurdly predicts a bright spot in the shadow's center. This is clearly impossible!"
Arago's experiment: The bright spot was observed exactly as predicted. Fresnel won the competition, proving light is a wave.

Why It Works

Light diffracts around the edges of the obstacle. The key insight:

All paths from the edge
to the shadow center
are EQUAL in length!

Since all these waves travel the same distance, they arrive in phase and constructively interfere.

The result: maximum brightness exactly where you'd expect complete darkness!

Modern Applications

1. Optical trapping of nanoparticles
2. High-precision alignment in lithography
3. Testing optical system quality
4. Studying wave phenomena

The spot also has fascinating "self-healing" properties—it reforms even if partially blocked!