Fresnel lenses are mostly thin sheets made of polyolefin materials. One side of the lens surface is smooth, and the other side is engraved with concentric circles from small to large.
The rings on the flash of the mobile phone come from a Fresnel lens. The purpose of the flash is to illuminate the subject in front. In order to make full use of the light emitted by the lamp tube, we need to focus these lights forward. For the light emitted to the back of the tube, people first think of using a concave reflector, but the light (yellow) that the concave reflector shoots toward the front and the side is only a small part, that is, the light that is really straight forward can be illuminated. The light from other directions was all lost when shooting the subject; although the later deepened mirror improved the problem of light emission, it greatly increased the thickness of the flash, and some light (indicated in blue) was still lost.
In pursuit of ultra-thin mobile phones, people use thinner Fresnel lenses instead of convex lenses. This magical lens is much thinner than convex lenses but can achieve the same optical path as convex lenses. In addition to being thinner, Fresnel lens also has less chromatic aberration. Therefore, many flashes (including mobile phones) will use a shallower reflective concave mirror and Fresnel lens to achieve the effect of "light from any direction converges forward".