Thin lens is the index of refraction of the lens material, and "R" and "R" are the radii of curvature of the two surfaces. For a thin lens, "d" is much smaller than one of the radii of curvature (either "R" or "R"). In these conditions, the last term of the Lensmaker's equation becomes negligible, and the focal length of a thin lens in air can be approximated by Here "R" is taken to be positive if the first surface is convex, and negative if the surface is concave. The signs are reversed for the back surface of the lens: "R" is
Parallel projection Parallel projection A parallel projection is a projection of an object in three-dimensional space onto a fixed plane, known as the "projection plane" or "image plane", where the "rays", known as "lines of sight" or "projection lines", are parallel to each other. It is a basic tool in descriptive geometry. The projection is called "orthographic" if the rays are perpendicular (orthogonal) to the image plane, and "oblique" or "skew" if they are not. A parallel projection is a particular case of "projection" in mathematics and "graphical projection" in technical drawing. Parallel projections can be seen as the limit of a