The POSITIVE AND NEGATIVE PRINCIPLE OF FRESNEL LENS
Positive Fresnel lens: The light enters from one side, and exits through the lens on the other side to focus into a point or exit as parallel light. The focus is on the other side of the light and is finitely conjugated. This type of lens is usually designed as a collimator lens (such as a Fresnel lens for projection, a magnifying glass, etc.) and a condenser lens (such as a heat-collecting and concentrating Fresnel lens for solar energy. Negatiive Fresnel lens, on the contrary, the focus and the light are on the same side, and the surface is usually coated as the first reflecting surface.