![]() ![]() ![]() The focal points and planes can be found for any optical system, except if it is an afocal optical system. The focal planes are defined as the planes perpendicular to the optical axis which contain the focal points. The back focal point (or rear focal point) is obtained by considering a situation where the input rays (on the left side) are parallel to the optical axis.īy again extrapolating the outgoing rays, one finds the back focal point (Figure 3).įor a defocusing system, the back focal point can be a virtual point located on the front side. Figure 2:įor a defocusing system, the front focal point can lie on the back side! ![]() It may be confusing that it is called a front focal point even though it is actually located on the back side. The ray paths within the device are only extrapolated from the outer ray paths.įor a defocusing system (Figure 2), the front focal point can be located on the back side this is a virtual focal point. The optical input is considered to be on the left side. The front focal point is related to parallel rays on the back side (= output side). Here, one can extrapolate the rays on the input side (left side) and finds that they meet in one point, which is the (real) front focal point (see Figure 1). In Gaussian optics, one defines various types of cardinal points, to which the focal points belong.įor the definition of the cardinal points, one does not consider the real light path in the optical system, but only the rays outside it, which can be extrapolated.įor defining the front focal point, we consider a situation where the output rays (right side) are parallel to the optical axis. How to cite the article suggest additional literature Encyclopedia > letter F > focal points and focal planes Focal Points and Focal Planesĭefinition: points to which parallel input rays are concentrated by an optical system, and the planes going through those points ![]()
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