What is the function of fresnel lens at projector?

Answer 1

A fresnel lens helps focus a beam of light going through it. Light bends (refracts) as it passes through different transparent materials, like through a glass ball, or when it goes from air into water (such as the surface of a pond or lake.)

What a fresnel lens does is to emulate the refraction of light through a glass ball, but in a compressed space. Picture taking a 15" diameter glass ball and cutting it into concentric circles, such as at diameters of 3", 6", 9", 12", and 15". Then take the slices that were cut apart and reassemble them on a flat surface. You will end up with a relatively thin glass assembly (maybe 3" thick) that has similar refraction properties of the original 15" thick glass ball.

This compressed reassembly is what makes up a fresnel lens...emulating refraction (and focusing) of light that otherwise would have required a much thicker lens.

At least...that's my layman's understanding of how it works...to save space from having a completely circular sphere for focusing. The most common use of a fresnel lens that I am aware of was in lighthouses, to focus and project the beam of light from the gas lamp (or whatever light source was being used in the lighthouse.)