Optical Board - Diverging Spherical Lens
To determine the focal point of a concave lens.
Optical board with diverging lens, convex and concave lens to produce parallel rays.
Parallel rays incident on a diverging plano-concave spherical lens appear to emanate from the focal point of the lens. Two diverging lenses can be placed together to increase the divergence and decrease the focal length. A slit baffle with concave and convex lenses are used to create the set of parallel rays, which can be varied in number and in spacing.
Alex J. Dragt, Lie Albegraic Theory of Geometrical Optics and Optical Aberrations, Journal of the Optical Society of America 72,372-379.