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Arago's Spot or Fresnel's, or Poisson's.   Is there a bright spot in the centre of a shadow?   An experiment crucis that helped decide the nature of light.

Optics experiments and images by Michiel de Boer (site).  Video.

At left a laser beam shines on a sphere. We are in its shadow looking towards the laser. The sphere’s dark side has a bright spot at its centre. In the above right image a steel sphere is held up by magnetic beads. That too has a bright spot on its dark side.

At least one eminent French academician said it should not exist. Common-sense says it is absurd. How can a bright spot exist in the centre of a shadow?

All images ©Michiel de Boer, shown with permission

The experiment at its very simplest.

Light waves diffract around the sphere's edge. It takes some complex maths to show that a spot forms.    

Newton's corpuscles would leave a completely dark shadow..

Michiel de Boer has kept the camera lens for this image but on an extension tube to grossly defocus the sphere itself to leave the diffraction pattern only.

In the centre is the Arago-Fresnel-Poisson spot.

The vertical striations are interference patterns from laser light scattered by the sphere support material.


The spot structure in a less than perfect world is complex.

Irregularities in the laser beam source also contribute to the diffraction patterns.

The sphere's dark side. The rim shines brightly by diffracted light.

In science, as in other activities, eminence and authority can carry great weight.

Isaac Newton in his landmark book ‘Opticks’ of 1704 proposed that light was tiny corpuscles travelling in straight lines. Newton hesitated over the decision. It struggled to explain already known interference fringes or diffraction. And across the North Sea was the scientific giant, Christiaan Huygens. He had in 1678 already published a rival theory that light was longitudinal waves (like sound). Huygens provided a reasonable explanation of refraction, interference and diffraction. Nonetheless, and in spite of mounting evidence, Newton's corpuscular theory held sway for the next hundred years and more.

It came to a competition. In 1818 the French Academy of Sciences launched one to settle the question - was light corpuscles or waves?

Engineer Augustin Jean Fresnel (lighthouse lenses) submitted his wave theory.   A brilliant scientist, Siméon Poisson was one of the judges. He was also an ardent supporter of corpuscular theory. He sought a flaw in Fresnels's paper. He extended Fresnel's analysis and deduced that wave theory predicted a spot of light directly behind a lit disk or sphere. An impossible and absurd result!   The spot was not visible and that was the end of wave theory.

Too clever! Shortly afterwards Dominique Arago, head of the judging committee, found the spot.

To be fair to Poissan, the experiment without lasers* is very difficult. Also, the occulting disk or sphere must be very smooth and round (witness the irregularities in the images here). Its placement must meet certain conditions for Fresnel diffraction to hold.

Physics advances come from unexpected quarters. Using Michael Faraday's experimental findings, James Clerk Maxwell in 1865 developed an elegant mathematical description of electricity and magnetism. His concise set of equations showed that light was transverse electromagnetc waves.

Quantum theory added another perspective. Depending on circumstances, we might interpret the world as particles or waves. An electron microscope suitably adjusted will show a very nice Arago spot!

A star is another source of parallel light and under very steady conditions a modern telescope with a central obstruction can show the spot if a bright star is racked well out of focus into a large disk of light.