What does a reflecting telescope use to bend light?

A reflecting telescope uses mirrors to redirect and focus light. Its large primary mirror gathers light from distant objects and reflects that light toward a focal point, where an eyepiece, camera, or scientific instrument can form and record the image. In everyday quiz language this is sometimes described as “bending” light, although the more precise optical term is reflection rather than refraction.

Most reflecting telescopes use a curved concave primary mirror. In a Newtonian design, a smaller flat secondary mirror sends the focused light sideways to the eyepiece. Other arrangements, including Cassegrain telescopes, use a secondary mirror to direct the light back through an opening in the primary mirror.

Isaac Newton built the first practical reflecting telescope in 1668. Mirrors became especially important for large astronomical instruments because a mirror can be supported from behind, while a massive glass lens must be supported around its edge and can sag under its own weight. Reflectors also avoid the chromatic aberration produced when lenses focus different colors at slightly different points. A refracting telescope, by contrast, uses lenses as its primary optical element.