What is light pollution?

Light pollution is unwanted light that is projected up into the night sky. Because it makes the sky much brighter than the natural night sky, it has significant impacts on astronomy–drowning out the light from faint objects like galaxies and nebulae and increasing the sky background for faint stars. It comes from poorly designed or misused light fixtures beaming light above the horizontal plane and represents wasted energy. For most purposes, light is needed on the ground and not in the sky.

Light pollution can be amplified by air pollution as dust and smoke scatter skybound radiation in all directions, further brightening the sky. Both kinds of pollution are environmental problems that affect not only astronomy, but animal life and human health as well.

Astronomers can avoid light pollution by traveling to dark-sky locations far away from cities (the source of most light and air pollution). To a limited extent, eyepiece and camera filters can be used to counteract light pollution.

See here: How does light pollution affect views through my telescope?