Light is both obvious and mysterious. We are covered in yellow warmth every day and dismiss the darkness with incandescent and fluorescentbulbs. But what exactly is light? We catch glimpses of its nature when a sunbeam angles through a dust-filled room, when a rainbow appears after astormor when a drinking straw in a glass of water looks disjointed. These glimpses, however, only lead to more questions. Does light travel as a wave, a ray or a stream of particles? Is it a single color or many colors mixed together? And what are some of the common properties of light, such as absorption, reflection, refraction and diffraction?
Over the centuries, our understanding of light has changed. Some of the earliest scientists and philosophers considered the true nature of this mysterious substance that stimulates sight and makes things visible. The first real theories about light came from the ancient Greeks. Many of these theories described light as aray-- a straight line moving from one point to another. Pythagoras, best known for the theorem of the right-angled triangle, proposed that vision resulted from light rays emerging from a person's eye and striking an object. Epicurus argued the opposite: Objects produce light rays, which then travel to the eye. Other Greek philosophers, most notably Euclid and Ptolemy, used ray diagrams quite successfully to show how light bounces off a smooth surface or bends as it passes from one transparent medium to another.
Around AD 1000, Arab physicist Alhazen concluded from his experimental observations that light actually travels into the eye. He observed that "the eye, when looking at a very strong light, feels pain and may be damaged." He also observed that the eye registers an afterimage after looking at a bright light. In 1604, Johannes Kepler introduced the idea that light is emitted by sources like the sun and is the reflected from object into the eye.
By the 17th century, some prominent European scientists began to think differently about light. One key figure was the Dutch mathematician-astronomer Christiaan Huygens. In 1690, Huygens published his "Treatise on Light," in which he described theundulatory theory. In this theory, he speculated on the existence of some invisible medium -- an ether -- filling all empty space between objects. He further speculated that light forms when a luminous body causes a series of waves or vibrations in this ether. Those waves then advance forward until they encounter an object. If that object is an eye, the waves stimulate vision.
Although this stood as one of the earliest wave theories of light, not everyone embraced it, including Isaac Newton. In 1704, Newton proposed a different idea in his great work
Opticks
, describing light as corpuscles, or particles and proposed that the different colors of light have different masses. Because of Newton's stature in the world of science, his findings stood for a hundred years until, in 1801, 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 thedouble-slit experimentand 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.
In the 1860s, Scottish physicist James Clerk Maxwell formulated the theory ofelectromagnetism. Maxwell described light as a very special kind of wave -- one composed of electric andmagneticfields. 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 aselectromagnetic 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).
In 1905, Albert Einstein suggested that the wave theory of light might be incomplete, and that light has some characteristics of particles. He studied thephotoelectric effect. First, he began by shiningultraviolet lighton the surface of a metal. When he did this, he was able to detect electrons being emitted from the surface. This was Einstein's explanation: If the energy in light comes in bundles, then one can think of light as containing tiny lumps, orphotons. When these photons strike a metal surface, they act like billiard balls, transferring their energy to electrons, which become dislodged from their "parent" atoms. Once freed, the electrons move along the metal or get ejected from the surface. The particle theory of light had returned. The quantum theory of light -- the idea that light exists as tiny packets, or particles, called photons -- slowly began to emerge.
Today, physicists accept the dual nature of light. In this modern view, they define light as a collection of one or more photons propagating through space as electromagnetic waves. Photons make it possible for us to see the world around us. In total darkness, our eyes are actually able to sense single photons, but generally what we see in our daily lives comes to us in the form of billions of photons produced by light sources and reflected off objects. If you look around you right now, there is probably a light source in the room producing photons, and objects in the room that reflect those photons. Your eyes absorb some of the photons flowing through the room, and that's how you see.
