Big thing to capture as many photons as possible: focuses light with lenses or mirrors. A detector of some kind is required to analyze or capture that light: your eye, photographic plates/film, ccds, etc. Taken together, a telescope is a series of lenses and/or mirrors that focuses (concentrates) light and puts that concentrated light on a detector. Two means to achieve the same goal: get light to the eyepiece and detector. The bending of light as it passes through a medium: air, water, glass, etc. The angle of refraction (how much it bends) is inversely proportional to the wavelength of light. Shorter wavelengths bent more than longer ones: blue and violet bent more than red. A lens is a specially designed medium (glass or plastic) to bend all incoming parallel light rays to a single point (focus) Focus: the point where all light rays converge. Focal length: the distance from lens to focus: for lenses, focal length depends on wavelength.