Telescopes are tools used by astronomers to see faraway objects. The first telescopes used pieces of curved, clear glass called lenses (refracting telescopes), but most telescopes today use mirrors (reflecting telescopes). Telescopes use these lenses or mirrors to gather the light better than our eyes can do on their own.
In a refracting telescope, the objective lens gathers and bends the light, then the eyepiece focuses and magnifies the light, enlarging the image for our eyes. The size of the image produced by the telescope depends on the curvature of the lenses. Lenses with different curvatures will change the magnification of the telescope, so thicker lenses make more powerful telescopes.