diffraction

When the shading object is small, its size can be compared with the wavelength.

Diffraction will occur.

The meeting of peaks is the bright spot.

The extreme dark spot where the peaks and valleys meet.

This will be Newton's ring.

Newton's rings will be mentioned in any book involving interference. This is an interference image, which comes from a hemispherical lens placed on a flat uncoated glass surface. This phenomenon is equivalent to what you see on two microscope slides. In this example, the interference image looks like a series of rings around the center. There may be some pictures of these rings in the textbook. Attached here is a schematic diagram, from which it can be seen that the intersection of the residual air and the intermediate air on the lens plane is guaranteed not to be within the scope of the legend. The reason is that if the interference fringes are visible to the naked eye, the radius of the lens surface must be very large.