Lenses have been made for thousands of years, with the oldest known artifact shown in Figure 4.8(a). It was constructed before 700 BC in Assyrian Nimrud. Whether constructed from transparent materials or from polished surfaces that act as mirrors, lenses bend rays of light so that a focused image is formed. Over the centuries, their uses have given rise to several well-known devices, such as eyeglasses (Figure 4.8(b)), telescopes, magnifying glasses, binoculars, cameras, and microscopes. Optical engineering is therefore filled with design patterns that indicate how to optimize the designs of these well-understood devices. VR headsets are unlike classical optical devices, leading to many new challenges that are outside of standard patterns that have existed for centuries. Thus, the lens design patterns for VR are still being written. The first step toward addressing the current challenges is to understand how simple lenses work.