Visible light

Visible light is by far the most familiar form of EM wave, simply because it is the one band the human eye can detect directly -- which is exactly why this narrow wavelength range earns the name 'visible light' in the first place. It is produced by the rearrangement of the outer orbital electrons of atoms and molecules as they move between energy levels (atomic excitation), for example in a gas-discharge tube.

Different wavelengths within the visible band are perceived as different colours, tabulated in Table 13.2 from violet (shortest wavelength, around 380-450 nm) through blue, green, yellow and orange to red (longest wavelength, around 620-750 nm). The visible light emitted or reflected from objects around us is our primary channel of information about those objects and about our surroundings generally.

An important practical consequence of where visible light sits in the spectrum concerns astronomy: radio waves and visible light are both able to pass through the Earth's atmosphere and reach the ground without being significantly absorbed, which is why radio telescopes and optical telescopes can both be built and operated from the ground. Every other band in the spectrum -- gamma rays, X-rays, ultraviolet, infrared, and microwaves -- gets absorbed to a much greater degree by atmospheric gases and dust, so telescopes designed to observe in those bands must be carried aboard artificial satellites and operated remotely from Earth. Even visible light, though it does reach the surface, has its intensity reduced somewhat by atmospheric absorption and scattering, which is why optical telescopes are still preferentially sited at high altitude …