Gaussian lens formula

Applet: Katie Dektar

Technical assistance: Andrew Adams

Text: Marc Levoy


In the preceeding applet we introduced Gauss's ray diagram, which allowed us to find for any point in object space the position in image space where rays leaving that point reconverge (i.e. come to a focus). One implication of Gauss's diagram is that points on a plane in object space lying parallel to the lens focus to positions on a plane in image space that is also parallel to the lens. Thus, we can talk about how distances in object space (to the left of the lens in the diagram below) relate to distances in image space (to the right of the lens). This relationship, which depends on the focal length f of the lens, is given by the Gaussian Lens Formula, shown in the lower-left corner of the applet below. The standard symbols for object space distance and image space distance are so and si, respectively.

It is beyond the scope of this applet to derive the Gaussian lens formula. Suffice here to say that it can be derived directly from Snell's law of refraction, making only a few assumptions along the way. The steps in this derivation, the geometrical constructions required, and some of the algebra can be found in slides 15-23 of this lecture, or in simplified form in slides 24-26 of the same lecture. A complete derivation can be found in any undergraduate textbook on optics, such as Chapter 5 of Hecht's Optics.