30-01-2013, 03:47 PM
Coherent light
Coherent light.docx (Size: 186.28 KB / Downloads: 23)
Holograms
A hologram is very much like a photograph, they both are recordings of light. Nonetheless, several distinct differences in the way light is recorded make them worlds apart. Unlike photographs, holograms are not recordings of simply intensities of light, but also recordings of its phase distribution. In order to do that, holograms must be recorded with coherent light. Not until the laser was invented was it possible to create a hologram.
When light waves hit an object, it is reflected, scattered, and refracted by the surface of the object, changing the phase of the waves. Different objects of different shapes will change the form of the waves differently, leaving a distinct fingerprint, or impression, of the object onto the light waves. Now, if someone tried to take a picture of a bullet, assuming that it is taken at the right moment, the image of the bullet will most likely come out as a blurry streak. Taking a picture of a light wave, which happens to be quite a bit faster than a bullet, will be hundreds of times harder, if not impossible. Luckily, there is another way. When two waves come into contact, they interfere with each other, the resulting interference pattern is called a standing wave. As its name implies, as long as the two light waves do not change, the standing wave will remain the same and "stand still." This is what is recorded onto a hologram.
With that in mind, a hologram is created by first splitting a laser beam into two. One is called the object beam and the other is called the reference beam. The object beam is shined at the object, which is then reflected and exposed to the film. The reference beam is exposed to the film without any change. At the film, the reference beam interferes with the object beam, creating an interference pattern, a standing wave, which is recorded onto the film.
Lasers
The word "laser" is an acronym for "light amplification by stimulated emission of radiation." What does it mean? In short, a high energy atom can be stimulated to release light that is in phase (in step, or matching) a wave that hits it, thus amplifying the stimulating wave. When billions and billions of atoms do this at once, they produce a monochromatic ray of light where all the waves are in step with each other. This is a great advantage in applications that require a focused and concentrated beam of light. In science too, the monochromatic nature of this light (being of a single wavelength) makes it easier to use in experiments.
When lasers were first invented in 1960, no one knew what to do with them. Today, you can find them almost everywhere, from the CD players, to supermarket barcode scanners, to surgery tools, to the Mars Sojourner Rover, which used laser rangefinding to avoid running into rocks. One application that promises to vastly improve the Internet you are using to view this site is fiber optics. Because of its high frequency, light can carry massive amounts of information. (Just compare what you see through your eyes with what you hear!) However, until the laser was invented, light usually spread out, diffused, and generally got blurred so that at large distances no information could be extracted. With a laser beam, the light is very bright, monochromatic, and tightly focused. This helps a laser beam carry for great distances. (Scientists bounced laser beams off the moon!) However, for earthly destinations, air, dust, and objects will often get in the way of a laser beam, preventing it from traveling too far. That is, until fiber optics were invented. Fiber optics are simply thin strands of ultra-pure glass. Due to the angle of incidence, there is total internal reflection for a laser beam traveling down this strand of glass, and thus, most of the light arrives intact at the other end, delivering vast amounts of information.
Interference
Interference is the interaction between waves traveling in the same medium. When two waves come into contact, depending on the phase differences along the waves, constructive and destructive interferences will occur.
In constructive interference, the amplitude of the wave is amplified. This happens when the two waves are in phase -- if the crests and troughs of the waves coincide with each other. Consider two waves, one with a crest of +1 units and coinciding with a wave of +2 units in amplitude at that point, traveling in opposite directions on the same medium. When these troughs come into contact, the resulting amplitude will be the sum of the two waves, which is +3 in this case. Once the waves pass each other, however, they will resume their original course with their original amplitude -- as if they have not been disturbed at all.
Destructive interference is very much like constructive interference except that the two waves cancels out each other. This happens when the waves are out of phase -- when the crests of one wave coincide with the troughs of the other. Consider two waves, one with a crest of +1 coinciding with a wave of -2 units in amplitude at that point, traveling in opposite directions on the same medium. At the point of contact, the resulting amplitude will be the difference of the two waves, which is -1 in this case. Just like constructive interference, once the waves pass each other, they will resume their original course with their original amplitude -- as if they have not been disturbed at all.
Fire and Light Bulbs
The most ancient of discoveries and the epitome of invention, fire and light bulbs both provide light by the same process: incandescence.
In fire, from forest fires to Bunsen burners, chemical reactions release heat, releasing gases and raising materials to high temperatures, where the gases and materials incandesce. Depending on the reactants, different temperatures are produced resulting in different colors. Also, certain chemicals can tint the color of the flame. In a Bunsen burner, the reactants are primarily methane (CH4) and oxygen (O2). The products are also simple: water (H20) and carbon dioxide (CO2). When you look at the flame from a Bunsen burner, you can see different sections with different colors. These differences are due to variations in heat (the hottest point is the tip of the inner cone) and also the chemicals being heated. In forest fires by contrast, the reactants are organic molecules in the wood. These are fairly complex with many portions being incombustible, so the product of the fire contains many impurities and the fire is much cooler. Thus, wood fires tend towards orange-yellow, or reddish when they die out.