# How did Arago find the Arago Spot without a laser?

Famously, Poisson showed that Fresnel's wave model of light would predict a bright spot in the very center of a circular shadow, which he interpreted as an absurd result. But Arago was able to perform the experiment, giving strong evidence toward the wave theory of light.

Nowadays, we can replicate the experiment with a laser and any small, sufficiently circular object.

But... Arago didn't have lasers in his time. The Wikipedia article (and general internet sources) says that he molded a metallic circle to a glass plate with wax, but mentions nothing about the light source. How did he generate a coherent light source, or if he didn't, how did he perform the experiment? (Is it even documented?)

## 2 Answers

How did he generate a coherent light source, or if he didn't, how did he perform the experiment? (Is it even documented?)

It is most important that the focusing object be round, next the light source must be small enough that the center of the projection area is not lit by incident rays - for example the Sun and the Moon do not produce this effect on the Earth's surface.

For an Aragon spot to be seen:

"The dimensions of the setup must comply with the requirements for Fresnel diffraction. Namely, the Fresnel number must satisfy

$$F={\frac {d^{2}}{\ell \lambda }}\gtrsim 1,$$

where

$d$ is the diameter of the circular object,

$ℓ$ is the distance between the object and the screen,

$λ$ the wavelength of the source.

Finally, the edge of the circular object must be sufficiently smooth.

These conditions together explain why the bright spot is not encountered in everyday life.

See Physics World's article: "The spot in the shadow":

"According to Fresnel’s work, if light were shone on a circular obstruction, a bright spot would appear in the centre of the shadow, as bright as if the obstruction were not there at all. Obvious nonsense! Not only that, Fresnel’s equations indicated that light shining through a circular hole could produce a dark spot in the middle.

The committee’s head was, however, François Arago – one of the few French scientists besides Fresnel acquainted with Young’s work, and therefore able to appreciate Fresnel’s contributions. Arago carried out the experiment with a flame, filters and a 2 mm metal disc attached to a glass plate with wax. To everyone’s surprise, and Poisson’s chagrin, Arago observed the spot and Fresnel won the competition.

...

If the demonstration is so simple, I asked Metcalf, why wasn’t it discovered earlier in things like eclipses? “The Moon’s not nearly round enough,” he snorted. “All those mountains! The Sun’s not a point source of coherent light. People didn’t always have laser pointers.” The critical point

The episode illustrates the ambiguities of discovery. Who’s the discoverer? Fresnel, who produced the original framework? Poisson, who showed the spot was a direct consequence but was firmly convinced that it didn’t exist? Arago, who did the experiment? Moreover, two other scientists turned out to have noticed the spot a century earlier but did not know what to make of it.

What about the French Academy, whose actions set the discovery in motion? Didn’t Young play a role? Even Newton? Moreover, the spot is just an illustration of a more general phenomenon that complementary obstruction patterns produce complementary diffraction results, described by Jacques Babinet’s theorem.".

My understanding is that the important characteristic of the light source is its size, not its coherence. The wikipedia article refers to a "point source". A laser is a good approximation to a point source.

But, I think the size of the source just hast to be small relative to the object blocking the light, so the requirements are not so stringent. (It would be difficult if the size of the source should be small relative to the wavelength.) You can make a simple point source by inverting a camera obscura. Basically a bright light in a box with one very small hole in one side will give you something which is a good enough point source for the Arago experiment.