Aristotle thought light to be "symbebekos," an accidental quality. Another Greek philosopher, Empedocles, opined light to be rays projected from the eyes to illuminate objects. Abu Ali al-Hasan, an Arabic scientist, demonstrated that light originates from the sun and is reflected off what we see. Galileo argued light emerges from the hearts of atoms—which is close (it is emitted by electrons when they lose energy). Isaac Newton said light is made up of corpuscles (particles) and proved it. Christian Huygens said that light is made up of waves, and Thomas Young proved it. The likes of Max Planck and Albert Einstein showed that both views were correct: light can be measured as a stream of particles (photons), or as the rippling motion through space of pure energy (waves). In 1865, James Clerk Maxwell published a set of mathematical equations that bound together what had been thought to be the separate phenomena of light, electricity, and magnetism. Light waves were shown to be composed of electrical and magnetic fields oscillating at right angles to one another and to the direction in which the waves are travelling. This physical unification gave us our current understanding of light as electromagnetic radiation.

Electromagnetic radiation can be categorized according to the energy of its photons or the length of its waves, a spectrum that extends from radio waves, with energies of less than a billionth of an electron volt per photon and wavelengths measuring more than 10,000 kilometers, to gamma rays, with energies topping a billion electron volts per photon at wavelengths less than 10 trillionths of a meter. Visible light, the electromagnetic radiation that human eyes are engineered to "see," comprises less than a millionth of one percent of this spectrum. Were scientific observations to be based solely on visible light, the view would be severely limited.

Ranging in wavelengths from 6,500 angstroms (red) to 4,000 angstroms (violet), visible light waves are simply too large to resolve images of atoms and molecules. No matter how high the magnification, visible light waves pass over atoms and molecules unaffected. It would be like trying to determine the size and shape of a tennis ball by observing its impact on the motion of ocean waves.

Beyond visible light on the electromagnetic spectrum, however, there lies an expanse bridging high energy ultraviolet and low energy x-rays, sometimes referred to as the XUV region. Wavelengths here range from 300 to 3 angstroms, ideal for studying and manipulating a wide number of atomic elements and the molecules they form.