 # L - Raytracing Reference

## Lathe

A lathe, originally, was a tool used to shape objects by shaving them as they turned round and round. If you have table legs with round cross-sections, they were probably made with a lathe. A computer graphics lathe object is used to model shapes just like this. Lathe objects are surfaces of revolution: they have round cross-sections and arbitrary longitudinal sections.

## Lens

A lens is a transparent object used to converge or diverge light rays through refraction. The most common example of a lens is a hand lens or magnifying glass. Spectacles use more complex lenses. Binoculars, telescopes and cameras use combinations of lenses.

There are two broad classes of lenses: convex (converging) and concave (diverging). These are illustrated below. ##### Top: a convex lens converges light raysBottom: a concave lens diverges light rays

The point through which a ray must pass if it remains parallel to its original direction after refraction is called the "optical centre" of the lens. The point to which a parallel beam of light converges after passing through a convex lens is known the "first principal focus" of the lens. The point from which a parallel beam of light appears to diverge after passing through a concave lens is known as the first principal focus of this lens. The "second principal focus", in both cases, is the point towards which a beam of light must converge if it is to be rendered parallel after passing through the lens. The distance from the optical centre to the first principal focus is called the "focal length" of the lens: it is positive for a convex lens and negative for a concave lens (hence the plus and minus powers of spectacle lenses). If an object is viewed through a lens, we observe that the position of the image of the object is different from that of the object itself -- it may be upside-down, or shifted forward or backward. The distance of the object from the optical centre is called the "object distance". The distance from the image to the optical centre is called the "image distance". All distances are positive when measured in the direction of the incident light and negative otherwise. Object distance (u), image distance (v) and focal length (f) are related according to the equation The "power" of a lens is the inverse of its focal length (1/f). It is usually measured in dioptres (1/metre).