The Poisson-Arago dot is a bright point of light hypothesised to appear at the centre of the shadow of a very round object that is illuminated with coherent light. This page, which explains Poisson's argument and shows the results of an experiment like that of Arago, is a support page for the multimedia tutorial Diffraction.
Poisson's argument against the wave nature of light
On the left: Siméon-Denis Poisson. The experiment Poisson proposed. On the right: Dominique-François-Jean Arago.
Thomas Young published the results of his famous interference experiment (Young's experiment) in 1807, strongly suggesting the wave nature of light. But if light is a wave, argued Siméon-Denis Poisson, then consider this experiment: A very round object is illuminated by light from the same source on its axis. Around the perimeter of the object, all points on the wave are in phase. In a Huygen's construction, draw a line from all of these points to the very centre of its shadow. All of these waves are in phase and so should yield constructive interference at this point. There should be a bright spot at the very centre of the shadow – at the very point where we might expect the shadow to be darkest. This, Poisson argued, was absurd and therefore the wave hypothesis must be wrong.
Enter Dominique-François-Jean Arago, on the right above. A good physicist, he knew that the only way to answer such a question was to do the experiment. Let's look at an experiment that has the important features of Arago's.
Arago's experiment and the result
We use a ball bearing as the very round object. Two needles connected to a magnet support it with little disturbance to the geometry. For a coherent light source, we use a laser with a lens to diverge the beam to make it a little larger than the ball bearing.
Our version of Arago's experiment: a ball bearing, a laser with a lens to diverge the beam and thus produce a relatively broad spot on the screen. Then we put the ball bearing in the beam. What will happen? (Click 'Play'.)
To the naked eye, the bright spot in the centre of the shadow is quite easy to see. The camera is less sensitive than our eyes, and in the raw video image the spot is less obvious, so we have brightened the whole of the close-up image, which is why white patches appear in the rings around the shadow. In the final frame of the clip, the bright object at far right is the light reflected from the close side of the ball bearing: you can see the needles either side. Next to this on the left we see the bright patch where the diverged laser beam strikes the screen. In its centre is the shadow of the ball bearing. To the left of that again we show an enlarged and uniformly brightened section of this image.
Arago had an adventurous and scientifically productive life. Much later in his life, Arago was president of the Exectutive Power Committee of France but, when Louis Napoleon demanded an oath of allegiance, he resigned.