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# The Poisson spot (Arago spot) and wave theory {#sec-resource-optics-poisson}

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**Are there measurements that reveal the propagation of secondary wavelets in the reverse direction, back towards the light source?**


Yes, there are experimental observations that demonstrate the propagation of secondary wavelets in the reverse direction, back towards the light source, as predicted by Huygens' wave theory.

One classic experiment that reveals this is the Poisson spot experiment, also known as Poisson's bright spot. In this experiment, a small opaque disk is placed in the path of a collimated beam of light. According to the ray theory of light, this should create a circular shadow behind the disk.

However, Huygens' wave theory predicted that there should be a bright spot in the center of the shadow, due to the interference of the secondary wavelets propagating backwards from the edge of the disk.

This bright spot, now known as Poisson's spot, was experimentally observed by François Arago in 1819, providing strong evidence for Huygens' principle of secondary wavelets propagating in all directions, including backward towards the light source.

Other experiments, such as those involving light passing through small apertures or around sharp edges, have also demonstrated the backward propagation of secondary wavelets, further confirming the wave-like nature of light as described by Huygens' theory.

[Arago spot explained.  It has a rich history and complex explanation. It seems that it depends on the the waves being truly spherical, i.e., heading back towards the source. ](https://en.wikipedia.org/wiki/Arago_spot#:~:text=In%20optics%2C%20the%20Arago%20spot,light%20behaves%20as%20a%20wave).

## Poisson Spot Experiment

The Poisson spot experiment is a classic demonstration that provides evidence for the wave nature of light, as predicted by Huygens' wave theory.

### Background

In the early 19th century, there was an ongoing debate between proponents of the wave theory of light (such as Huygens) and the particle theory of light (such as Newton). One key experiment that helped resolve this debate was the Poisson spot experiment, performed by François Arago in 1819.

### Experimental Setup

The setup for the Poisson spot experiment is as follows:

1. A small opaque disk is placed in the path of a collimated beam of light.
2. According to the ray theory of light, this should create a circular shadow behind the disk.

### Observed Result

Contrary to the ray theory's prediction, Arago observed a bright spot in the center of the shadow behind the disk. This bright spot is now known as the Poisson spot.

### Explanation using Huygens' Wave Theory

Huygens' wave theory of light provides an explanation for the Poisson spot:

1. According to Huygens' principle, each point on the wavefront acts as a source of a secondary spherical wavelet.
2. These secondary wavelets propagate in all directions, including backwards towards the light source.
3. At the center of the shadow, the backwards-propagating wavelets from the edge of the disk interfere constructively, creating the bright Poisson spot.

This backward propagation of secondary wavelets is a key prediction of Huygens' wave theory and is not explained by the ray theory of light.

### Significance

The observation of the Poisson spot provided strong experimental evidence in favor of the wave theory of light, and helped establish Huygens' wave theory as a more accurate model of light propagation compared to the prevailing particle theory.

This experiment is considered a landmark in the history of optics and the understanding of the nature of light.

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