Astronomy and the optical spectrum. The optical range of the electromagnetic spectrum runs from wavelengths of 310 millionths of a meter (nanometer, nm) to 1100 nanometers. The human eye sees wavelengths of 390 to 700 nm as colors ranging from violet to red. Interstellar gas produces bright emission lines at wavelengths unique to the elements in that gas, the most prominent coming from neon (Ne), hydrogen (H), oxygen (O) & sulphur (S). Optical telescopes are placed on mountains or in space to avoid most of the blurring of light caused by the atmosphere. Optical sources include stars, galaxies (from top left to top right) and gaseous nebulae (at upper center).