All telescopes rely on some ability to focus light to a point. The two ways are refraction and reflection. Here we describe what they are in the context of telescopes. This is part of my intro Astronomy class taught at Willam Paterson University and CUNY Hunter. Galileo's telescope Why is light slower in glass? - Sixty Symbols Richard Feynman - The Character of Physical Law - Part 1 Richard Feynman - QED: Photons -- Corpuscles of Light 0:00 Introduction 0:01 Telescopes 0:37 An Elementary Telescope 2:53 Refraction with a Prism 6:02 Snell's Law of Refraction 9:42 Focusing Light using Refraction by a Lens 12:46 Chromatic Aberration 13:48 Reflection off a shiny, smooth surface 14:12 Focusing Light using a Reflecting Mirror A properly curved mirror will bring the light to a focus.