I haz deth rey!

[230RN]

Guesses and questions from a techno-dummy: Does the frez lens spread the light over the entire convex surface, thereby concentrating the energy? Could that magic hotspot be focused onto a mirror and therefore redirected as a "spot" of intense energy?

All the parallel rays of light hitting the fresnel lens get bent so they all get together (are concentrated, converge) on one spot at the focal distance.

This is the same as a regular lens, except they brought the curved surface of a regular lens down to a flat surface with concentric rings, each ring reproducing a "ring" of the original lens:

http://80.36.73.149/almacen/medicina/oftalmologia/enciclopedias/duane/graphics/figures/v1/0520/001f.gif


Another example of how the curvatures of a regular lens are brought down to a flat surface:

http://off-camera-flash.com/wp-content/uploads/2014/08/Fresnel-lens-01.jpg





The simplest mechanics of a lens, that is, two prisms slapped together as a "lens":

http://www.niiler.com/phy140/doubleprism.png

Incidentally, there are Fresnel mirrors, too.  The last versions of the Polaroid insta-cameras had a Fresnel mirror in them.

Terry

(Credits in URLs.)

[HankB]

Another example of how the curvatures of a regular lens are brought down to a flat surface:

http://off-camera-flash.com/wp-content/uploads/2014/08/Fresnel-lens-01.jpg

Very rarely will the individual facets of a Fresnel lens be progressively curved as illustrated here - usually the pitch will be fixed at a set number of grooves per inch and the facet tilt will vary to approximate a surface which is tangential to the curved surface at that fractional aperture. This limits the potential image quality.

Of course, the risers will also have a little bit of draft built in to facilitate manufacture . . . and they may be "frosted" to produce a "low glare" Fresnel.

[230RN]

^ True.  Bob F. asked about it. Trying to keep it basic.