Symmetry in a Perturbed Optical System
  This paper investigates the effects that perturbations to an optical system, such as translations or rotations of the optical elements, have on the final location where a light ray strikes a detector. Symmetry arguments are employed to give selection rules for the combinations of those perturbations that can affect the light ray. It is shown that a ``Transverse Parity'' can be assigned to each type of perturbation and that only combinations with an overall odd parity will produce an effect. Some examples of how these results can be applied to undergraduate classroom or laboratory courses are given. 
Jason H. Steffen