## Light: Particle or Wave?

Jun 01, 2006
By Spectroscopy Editors
Volume 21, Issue 6

Is light a particle or a wave? Not only was this an important question for the modern understanding of the world around us, but the answer was long in coming.

Particles and Waves

 Figure 1: One property of matter is that no two pieces of matter can fit in the same point in space. This momentum-based toy, called Newton's cradle, is based on that property.
It should be obvious that matter — anything that has mass and takes up space — is ultimately particulate. In fact, matter defines what it is to be a particle. Particles come in pieces that can be broken into smaller pieces, and smaller pieces can be fused together to make larger pieces. No two pieces can occupy the same exact point in space, a concept that applies at the atomic and molecular level for matter that is a solution, a liquid, or a gas. Matter in motion tends to move in a straight line unless some force acts on it, and matter in motion tends to follow Newton's laws of motion (which can be reformulated into Lagrange's or Hamilton's laws of motion). Moving matter can be blocked by the presence of other matter, much like an umbrella blocks the paths of raindrops falling on it. Moving matter also can bounce off of surfaces, as demonstrated by many a basketball player. See Figure 1.

 Figure 2: Waves have certain properties that matter does not: wavelength, frequency, interference, and other properties. Permission to reprint courtesy Dr. Andrew Davidhazy, School of Photographic Arts and Sciences, Rochester Institute of Technology.
Waves behave differently. Waves (mechanical waves, at least) are a regular disturbance of some medium through which the wave travels and transfers energy. The particles of the medium oscillate regularly either in the direction of the wave (longitudinal waves) or perpendicular to the direction of the wave (transverse waves). A particular wave can be described by its wavelength, the distance between two similar amplitudes of consecutive waves, and its frequency, the number of waves passing a fixed point in space per second. The product of the wave's frequency and wavelength is the wave's velocity. Waves can combine, a process called interference. If similar amplitudes of two waves combine to make a larger wave, the waves are constructively interfering. If opposite amplitudes of two waves combine to cancel each other out, the waves are destructively interfering. Waves can diffract: they can change their direction in their medium, which is why you can hear someone yelling around a corner. Waves can be reflected from surfaces (think of an echo you might hear off of a far building). See Figure 2.

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