Why do refracting telescopes use lenses?

Refracting telescopes, or refractors, use lenses because they rely on the principle of refraction to gather and focus light. Refraction is the bending of light as it passes from one medium to another, such as from air into glass.1

In a refractor, the objective lens collects light from a celestial object and causes those light rays to converge at a focal point. A second lens, the eyepiece, is then used to magnify this image for the observer. By using lenses to increase the size of the light-gathering opening beyond that of the human pupil, these telescopes allow much more light to pass through, enabling the observation of smaller and fainter objects.12

While lenses are fundamental to the design of refractors, they can introduce chromatic aberration—a distortion where light of different colors is bent differently, potentially causing colored fringes around objects. To minimize this, modern lens designs often incorporate multiple components made from different types of glass.12