What is arago spot?

Arago spot, also known as the Poisson spot, is a bright spot that appears at the center of a circular shadow when a beam of light is diffracted around an opaque object. It was discovered by the French physicist François Arago in 1819 and was one of the earliest observed instances of wave-particle duality in light. Arago noted that the spot is still visible even if the diffracting object is gradually reduced in size until it is no longer visible to the naked eye. This was a key insight in the development of the wave theory of light and the understanding of the diffraction of light waves around objects. Today, the Arago spot is still a subject of fascination and study for physicists and may have practical applications in fields such as microscopy and lithography.