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For centuries, scientists debated the nature of light. Some claimed that light was a wave, behaving like a ripple in a pool. The opposing view was that light was a particle, like the droplets of water that flow from a kitchen faucet. Just when a prevailing view gained momentum, evidence for the other caused confusion. Finally, in the early 20th century, Albert Einstein called a tie: Light is both wave and particle. Those who believed in the particle theory of light followed Sir Isaac Newton. He described light as a series of particles, using a prism to prove his theory. To Newton, the clarity and sharpness of the prism shadows meant that light traveled as a shower of particles, each following a straight line until disturbed. Those who opposed Newton’s theory followed scientist Christiaan Huygens, who cited light’s diffraction and interference as proof that it is a wave. Diffraction, the bending of light as it passes around an object, and interference, when waves combine to form greater or lesser amplitude, occur in other mediums with wave-like properties, such as sound and water. Astronomers studying moving galaxies proved that light follows the Doppler Effect, the name for the change in sound as waves from the source move closer or farther away from you, elongating as they move away and shortening as they come closer. Visible light, as seen in the colors of the rainbow, exhibits similar properties, with longer wavelengths appearing as a red shift and shorter wavelengths as a blue shift. Until the turn of the century, this overwhelming evidence convinced most scientists that light was a wave, until Albert Einstein settled the score.