Telescope lenses gather extra light for the eye
Before Galileo, our ability to explore the universe was restricted by the amount of light the unaided eye could gather.We cannot see things in the far distance because the light that enters the eye from them is insufficient to activate the detector cells in the retina.
The objective lens of a telescope gathers extra light from distant objects and passes a much brighter beam to the eye. Things that are invisible to the naked eye become visible through a telescope.