A lens is an optical element that can refract light. When a beam of light passes through a lens, it will be refracted and focused, thus forming a light spot or image. However, the shape of the lens and the incident angle of light will affect the imaging and cause image distortion.

In radial distortion, when light passes through different parts of the lens, its refraction angle is different, which leads to image distortion. For a circular spot, when it passes through a certain type of lens, the bending of the lens will lead to different refraction of light at different positions, which will cause radial distortion of the image and eventually produce a square spot.