PURPOSE: Demonstrate on-axis astigmatism.

DESCRIPTION: On-axis astigmatism is the type which occurs in the eye. Light from a bright point source is focused onto a pinhole, which acts as the object for the system. The pinhole object is then focused by an "astigmatic lens" consisting of a 10 cm focal length spherical convex lens and a 30 cm focal length cylindrical convex lens. The vertical focus is a horizontal line and the horizontal focus is a vertical line; between these two foci is the "best" focus circle of confusion. The foci are shown by moving a ground glass screen along the optic axis, and can be seen as follows: Keep the screen perpendicular to the optic axis while moving it along the optic axis to see (moving outward from the astigmatic lens from left to right in the photographs below) (1) the beam immediately following the lens, (2) the horizontal focus (a vertical line), (3) the waist, (4) the vertical focus (a horizontal line), and (5) the expanding beam following the second focal point.

SUGGESTIONS: See Question of the Week #232 for information on using this demonstration to enhance class involvement.

EQUIPMENT: Bright point source with 10cm lens, pinhole, 10cm focal length spherical convex lens and 30cm focal length cylindrical convex lens, with ground glass screen, on portable optical rail cart.

SETUP TIME: 5 min.