Fresnel Lens

Using our thin-lens approximation, it should in principle not matter at all how thick a lens is.
Even with real lenses, one has only very small effects of the thickness, d, of the lens on the trajectory of the light ray passing through the lens.
Using this, one can construct Fresnel lenses. These are lenses made of segments consisting of circular concentric trapezoids and a circular end prism. The figure here shows a cross section through a Fresnel lens.

The advantages of Fresnel lenses are:
- They are thinner and use less material
- They are less heavy
- They absorb less light
- Thus Fresnel lenses are used in many optical appliances, such as overhead projectors.
Disadvantages: Since the thin-lens approximation is not exactly valid, one therefore obtains typical rings (Fresnel rings) on the image by using Fresnel lenses.