If you want to look it up, a lens like this is called a Fresnel lens [0]. You can imagine it as a 'normal' lens, without unnecessary bulk of the material, just

If you want to look it up, a lens like this is called a Fresnel lens [0]. You can imagine it as a 'normal' lens, without unnecessary bulk of the material, just keeping the curved surface and a flat backside here. Since this isn't possible while retaining the smooth curvature of a normal lens, it is stepped and has concentric rings of different curvatures instead. You can find one in front of the flash LED of your smartphone for example. [0] https://en.wikipedia.org/wiki/Fresnel_lens https://en.wikipedia.org/wiki/Fresnel_lens