Do you know what light means ?????

Light is at once both obvious and mysterious. We are bathed in yellow warmth every day and stave off the darkness with incandescent and fluorescent bulbs. 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 a storm or 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? Does it have a frequency like sound? And what are some of the common properties of light, such as absorption, reflection, refraction and diffraction?

You might think scientists know all the answers, but light continues to surprise them. Here’s an example: We’ve always taken for granted that light travels faster than anything in the universe. Then, in 1999, researchers at Harvard University were able to slow a beam of light down to 38 miles an hour (61 kilometers per hour) by passing it through a state of matter known as a Bose-Einstein condensate. That’s almost 18 million times slower than normal! No one would have thought such a feat possible just a few years ago, yet this is the capricious way of light. Just when you think you have it figured out, it defies your efforts and seems to change its nature.

Still, we’ve come a long way in our understanding. Some of the brightest minds in the history of science have focused their powerful intellects on the subject. Albert Einstein tried to imagine what it would be like to ride on a beam of light. “What if one were to run after a ray of light?” he asked. “What if one were riding on the beam? … If one were to run fast enough, would it no longer move at all?”

Einstein, though, is getting ahead of the story. To appreciate how light works, we have to put it in its proper historical context. Our first stop is the ancient world, where some of the earliest scientists and philosophers pondered the true nature of this mysterious substance that stimulates sight and makes things visible.
Over the centuries, our view of light has changed dramatically. The first real theories about light came from the ancient Greeks. Many of these theories sought to describe light as a ray — 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.

Arab scholars took these ideas and honed them even further, developing what is now known as geometrical optics — applying geometrical methods to the optics of lenses, mirrors and prisms. The most famous practitioner of geometrical optics was Ibn al-Haytham, who lived in present-day Iraq between A.D. 965 and 1039. Ibn al-Haytham identified the optical components of the human eye and correctly described vision as a process involving light rays bouncing from an object to a person’s eye. The Arab scientist also invented the pinhole camera, discovered the laws of refraction and studied a number of light-based phenomena, such as rainbows and eclipses.

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 the undulatory 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.

This stood as one of the earliest, and most eloquent, wave theories of light. Not everyone embraced it. Isaac Newton was one of those people. In 1704, Newton proposed a different take — one describing light as corpuscles, or particles. After all, light travels in straight lines and bounces off a mirror much like a ball bouncing off a wall. No one had actually seen particles of light, but even now, it’s easy to explain why that might be. The particles could be too small, or moving too fast, to be seen, or perhaps our eyes see right through them.

As it turns out, all of these theories are both right and wrong at once. And they’re all useful in describing certain behaviors of light.
Imagining light as a ray makes it easy to describe, with great accuracy, three well-known phenomena: reflection, refraction and scattering. Let’s take a second to discuss each one.

In reflection, a light ray strikes a smooth surface, such as a mirror, and bounces off. A reflected ray always comes off the surface of a material at an angle equal to the angle at which the incoming ray hit the surface. In physics, you’ll hear this called the law of reflection. You’ve probably heard this law stated as “the angle of incidence equals the angle of reflection.”

Of course, we live in an imperfect world and not all surfaces are smooth. When light strikes a rough surface, incoming light rays reflect at all sorts of angles because the surface is uneven. This scattering occurs in many of the objects we encounter every day. The surface of paper is a good example. You can see just how rough it is if you peer at it under a microscope. When light hits paper, the waves are reflected in all directions. This is what makes paper so incredibly useful — you can read the words on a printed page regardless of the angle at which your eyes view the surface.

Refraction occurs when a ray of light passes from one transparent medium (air, let’s say) to a second transparent medium (water). When this happens, light changes speed and the light ray bends, either toward or away from what we call the normal line, an imaginary straight line that runs perpendicular to the surface of the object. The amount of bending, or angle of refraction, of the light wave depends on how much the material slows down the light. Diamonds wouldn’t be so glittery if they didn’t slow down incoming light much more than, say, water does. Diamonds have a higher index of refraction than water, which is to say that those sparkly, costly light traps slow down light to a greater degree.

Lenses, like those in a telescope or in a pair of glasses, take advantage of refraction. A lens is a piece of glass or other transparent substance with curved sides for concentrating or dispersing light rays. Lenses serve to refract light at each boundary. As a ray of light enters the transparent material, it is refracted. As the same ray exits, it’s refracted again. The net effect of the refraction at these two boundaries is that the light ray has changed directions. We take advantage of this effect to correct a person’s vision or enhance it by making distant objects appear closer or small objects appear bigger.

Unfortunately, a ray theory can’t explain all of the behaviors exhibited by light. We’ll need a few other explanations, like the one we’ll cover next.
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.
Once Maxwell introduced the concept of electromagnetic waves, everything clicked into place. Scientists now could develop a complete working model of light using terms and concepts, such as wavelength and frequency, based on the structure and function of waves. According to that model, light waves come in many sizes. The size of a wave is measured as its wavelength, which is the distance between any two corresponding points on successive waves, usually peak to peak or trough to trough. The wavelengths of the light we can see range from 400 to 700 nanometers (or billionths of a meter). But the full range of wavelengths included in the definition of electromagnetic radiation extends from 0.1 nanometers, as in gamma rays, to centimeters and meters, as in radio waves.

Light waves also come in many frequencies. The frequency is the number of waves that pass a point in space during any time interval, usually one second. We measure it in units of cycles (waves) per second, or hertz. The frequency of visible light is referred to as color, and ranges from 430 trillion hertz, seen as red, to 750 trillion hertz, seen as violet. Again, the full range of frequencies extends beyond the visible portion, from less than 3 billion hertz, as in radio waves, to greater than 3 billion billion hertz (3 x 1019), as in gamma rays.

The amount of energy in a light wave is proportionally related to its frequency: High frequency light has high energy; low frequency light has low energy. So, gamma rays have the most energy (part of what makes them so dangerous to humans), and radio waves have the least. Of visible light, violet has the most energy and red the least. The whole range of frequencies and energies, shown in the accompanying figure, is known as the electromagnetic spectrum. Note that the figure isn’t drawn to scale and that visible light occupies only one-thousandth of a percent of the spectrum.

This might be the end of the discussion, except that Albert Einstein couldn’t let speeding light waves lie. His work in the early 20th century resurrected the old idea that light, just maybe, was a particle after all.