Lenses

Ray tracing is the technique of determining the paths light rays take; often thin lenses (the light ray bending only once) are assumed.

The thin lens equation relates the object distance d_o, image distance d_i, and focal length f.

A compound lens is an array of simple lenses with a common axis.

The lensmaker's formula is used to relate the radii of curvature, the thickness, the refractive index, and the focal length of a thick lens.

Because the index of refraction of a lens is greater than air, a ray moves towards the perpendicular as it enters and away as it leaves.