Light as Waves
Unlike water waves, light waves follow more complicated paths, and they don't need a medium to travel through.
When the 19th century dawned, no real evidence had accumulated to prove the wave theory of light. That changed in 1801 when Thomas Young, an English physician and physicist, designed and ran one of the most famous experiments in the history of science. It's known today as the double-slit experiment and requires simple equipment -- a light source, a thin card with two holes cut side by side and a screen.
To run the experiment, Young allowed a beam of light to pass through a pinhole and strike the card. If light contained particles or simple straight-line rays, he reasoned, light not blocked by the opaque card would pass through the slits and travel in a straight line to the screen, where it would form two bright spots. This isn't what Young observed. Instead, he saw a bar code pattern of alternating light and dark bands on the screen. To explain this unexpected pattern, he imagined light traveling through space like a water wave, with crests and troughs. Thinking this way, he concluded that light waves traveled through each of the slits, creating two separate wave fronts. As these wave fronts arrived at the screen, they interfered with each other. Bright bands formed where two wave crests overlapped and added together. Dark bands formed where crests and troughs lined up and canceled each other out completely.
Young's work sparked a new way of thinking about light. Scientists began referring to light waves and reshaped their descriptions of reflection and refraction accordingly, noting that light waves still obey the laws of reflection and refraction. Incidentally, the bending of a light wave accounts for some of the visual phenomena we often encounter, such as mirages. A mirage is an optical illusion caused when light waves moving from the sky toward the ground are bent by the heated air.
In the 1860s, Scottish physicist James Clerk Maxwell put the cherry on top of the light-wave model when he formulated the theory of electromagnetism. Maxwell described light as a very special kind of wave -- one composed of electric and magnetic fields. The fields vibrate at right angles to the direction of movement of the wave, and at right angles to each other. Because light has both electric and magnetic fields, it's also referred to as electromagnetic radiation. Electromagnetic radiation doesn't need a medium to travel through, and, when it's traveling in a vacuum, moves at 186,000 miles per second (300,000 kilometers per second). Scientists refer to this as the speed of light, one of the most important numbers in physics.