These web pages are a slightly updated revival of the original KOJAC web pages, that don’t exist anymore on the web site of the Applied Optics Group of IMT (formerly at the University of Neuchâtel, now at EPFL).
Plano-convex lenses present a curved interface and a planar interface. They are for instance the microlenses situated on top of a wafer, but can also encountered in macroscopic optical systems. Their asymetrical nature influences their properties of bending the light. Depending on their role in an optical system, their orientation has to be taken into account.
On axis, the main aberration is the spherical aberration. A common mistake is not to care about the lens orientation in the system. However, if the purpose is the focusing of parallel rays (a plane wave), the aberrations can vary of a factor four, depending on this “detail” (n=1.5 in the equation n3/(n3 - 2n2 +2)). The curved surface should be placed first on the light’s path, as you can experiment in the following applet.
This difference can be understood simply: As you can see in the previous applet, if the light falls upon the planar interface, it is not bent, and the whole effect will be done by the curved interface. On the other way, the whole lens effect is shared between the two interfaces. Their “effort” is smaller for each interface, so the aberrations are smaller.
Symetrically, the collimating set-up requires the lens to be placed the other way around. The following applet demonstrates this principle.
For both situations, the good position is the one where the center of curvature of the lens is in the direction of the focus.