visible light

Visible light (or light) is electromagnetic radiation (EMR) that our eyes can sense, i.e., visible to us. Cited wavelength ranges vary somewhat: a representative example is 390 to 750 nm, corresponding to EMR frequencies between 400 and 769 THz. The colors we sense depend upon the wavelengths of the light. EMR of longer wavelength than visible light is termed infrared (having wavelengths longer than that of red light) and shorter than visible light, ultraviolet (with wavelengths shorter than that of violet light). Visible light is a tiny portion of the spectrum of EMR around us, with a much smaller wavelength-range than the other well-known categories (ultraviolet, radio, etc.), but is naturally of particular interest to us since we sense it. The brightest portion of the Sun's spectrum is visible light, one reason our eyes are adapted to use it, and it falls within an Earth atmospheric window, i.e., the atmosphere is transparent to it; atmosphere blocks much of the rest of the electromagnetic spectrum from reaching us from space. The word optical is often used to indicate visible light when describing or naming an astronomical survey, observation, instrument, or telescope, but is also commonly used for the portions of infrared and ultraviolet for which equipment (e.g., telescope mirrors) is identical or similar.

Visible-light observation (optical astronomy) was the entirety of astronomy for millennia and it has only been in the 20th century that astronomy began to do more, largely beginning with the discovery of radio sources, and with the use of visible-light telescopes to image the portions of the ultraviolet and infrared adjacent to visible light. Visible light is also the primary type of observation by amateur astronomers, and casual astronomical viewing, such as back-yard astronomy. In the past, astronomy was basically the study of the visible night sky, with the pattern of relative movements of the stars, planets, and Moon, and their positions relative to the Sun. Weather has always been a challenge to visible-light viewing and light pollution is now a serious challenge. Viewing from high altitudes offer advantages, for optical astronomy as well as most other types of astronomical observation.