Thin lens positive if the surface is concave, and negative if it is convex. This is an arbitrary sign convention; some authors choose different signs for the radii, which changes the equation for the focal length. Certain rays follow simple rules when passing through a thin lens, in the paraxial ray approximation: By tracing these rays, the relationship between the object distance "s" and the image distance "s′" can be shown to be which is known as the thin lens equation. In scalar wave optics a lens is a part which shifts the phase of the wave-front. Mathematically this can be understood
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