In astronomy, the optical window is that portion of the electromagnetic spectrum that passes through the atmosphere all the way to the ground. Most EM energy never makes it through the atmosphere, so this is like a window that lets in just a little of what is out there. It is called "optical" because the wavelengths we can see are all in this range. The window runs from around 300 nanometers (ultraviolet-C) at the short end up into the range the eye can use, roughly 400-700 nm and continues up through the visual infrared to around 1100 nm, which is thermal infrared.

There is another absorption window called the "radio window" that lets through some (not all) radio waves. The radio window runs from about one centimeter to about eleven-meter waves.

In medical physics, the optical window is the portion of the visible and infrared spectrum where living tissue absorbs relatively little light. This window runs approximately from 650 nm to 1200 nm. At shorter wavelengths light is strongly absorbed by hemoglobin in blood, while at longer wavelengths water strongly absorbs infrared light.