This tutorial explores lens action in a plano-concave lens when the object is facing the flat face of the lens. To operate the tutorial, place your mouse cursor on the soldier, then click on the left-hand button and move him back and forth to view different levels of magnification.

For the purposes of this tutorial, we apply the following equation to describe lens action:

where f is the focal length of the lens, p is the distance of the distance with respect to the optical center of the lens, and q is the distance of the image to the optical center of the lens.

The flat surface of a plano-concave lens faces the object being imaged, and this results in a negative value for the focal length. The lens equation, in this instance, yields a negative image distance resulting in the image being formed on the object side of the lens. Therefore, the soldier is formed by an extrapolation of the light rays into a virtual image formed on the incoming-light side of the lens.

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