VISIBLE LIGHT

The visible portion of the EM spectrum lies within the wavelength range of 770 to 390 nm. Emissions at the longest wavelengths appear red; as the wavelength decreases, we see orange, yellow, green, blue, indigo, and violet in that order.

Visible light is transmitted fairly well through the atmosphere at all wavelengths. Scattering increases toward the blue, indigo, and violet end of the spectrum. This is why the sky appears blue during the daytime. Longwavelength light is scattered the least; this is why the Sun often appears red or orange when it is on the horizon. Red is the preferred color for terrestrial line-of-sight laser communications systems for this same reason. Rain, snow, fog, smoke, and dust interfere with the transmission of visible light through the air.