Answer
Huygens's principle states that every point on a wavefront can be considered as a point source of secondary wavelets, which then combine to form the new wavefront. It is a way of understanding the behavior of light waves as they propagate through space.
Work Step by Step
When light passes through a circular aperture, such as a lens or a pinhole, a diffraction pattern is formed on a screen placed beyond the aperture. This pattern is called the Airy pattern, and it consists of a central bright spot (the Airy disk) surrounded by a series of concentric rings.
However, the center of the Airy pattern has a dark spot, called the Poisson spot. This dark spot is a result of the wavelets from the edges of the aperture arriving at the center of the pattern with a phase difference. Since these wavelets are out of phase, they tend to cancel each other out at the center of the pattern, resulting in a dark spot.
In summary, the Poisson spot is expected because of Huygens's principle. According to this principle, every point on a wavefront can be considered as a point source of secondary wavelets. When light passes through a circular aperture, a diffraction pattern is formed on a screen placed beyond the aperture, and the center of this pattern, the Airy disk, has a dark spot, due to the wavelets from the edges of the aperture arriving at the center of the pattern with a phase difference, resulting in a dark spot.
