A bright spot appearing in the exact center of the shadow of a perfectly circular, opaque obstacle when illuminated by a point source or a highly coherent laser beam. It occurs because the diffracted waves from the circular edge all travel the same distance to the center of the shadow, interfering constructively.
Heard on the job
When aligning the spatial filter, you can sometimes see an Arago spot if a dust particle lands exactly in the center of the beam path.
Insider tip
It is famously known as the Poisson spot, named after the physicist who predicted it to mock the wave theory of light, only for Arago to experimentally prove it existed.
Also heard as
- Poisson spot