The Apple, the Cat and the Barometer
Action, Camera, Light
It is morning and time to get to work. As I rush out, I realize I cannot find my wallet. I look all over, many many times. Like many middle-aged men, I cannot find things, even when they are staring at me. Women do not seem to have this problem. But I have good company, as this problem seemed to have plagued the great Greek thinkers, notably Euclid and Plato. Finally, after what seems like an eternity of searching I find the wallet on my desk, a place I have searched umpteen times. How could I not have seen it before?
Luckily for me, Euclid and Plato have an explanation. According to them, there is a fire in our eyes. This fire generates a sort of “ray” and as we move our eyes around this ray falls on objects, and causes those objects to become visible. If the rays happen not to fall on a particular object, you cannot see it. Unluckily for me, Aristotle debunked this theory. If the ray theory was true, said Aristotle, then we would have no trouble seeing in the dark.
The eye sees light. How the eye works evaded scientists for a long time, till Leonardo da Vinci figured it out. He realized the eye casts an image behind it, just like a pinhole on a wall of a dark room creates an upside down image of the outside, on the opposite wall. Later, Descartes cut up eyes from animals and realized da Vinci was right.
As light enters the lens of the eye, it is refracted to produce an image of the scene in front of the eye upon the retina. The retina is covered with two kinds of cells, called “rods” and “cones”. The rods respond to the intensity of light and generate an electrical signal, whose strength is proportional to the amount of light falling on the rod. The optic nerves carry the signals from 120 million rods to the brain. The brain “processes” these signals and thus we see. If we only had rods, we would see the world in black and white (intensity of light). So we have cones, about 6 million of them. Some cones respond to red light, others to green and yet others to blue light. From the signals generated by the cones the brain adds color to our vision.
Light not only causes vision, it plays an important part in triggering emotions. The lovely hues of a spectacular sunset evoke complex emotions of happiness, awe, intrigue and serenity in humans. A fast action movie raises the blood pressure and creates apprehension. A gloomy day depresses people, while sunny days make them cheerful. A dark and empty alley can cause intense fear.
What exactly is light? The early Greeks thought light was some kind of particles. Then in the 1600’s due to experiments with diffraction grids, some scientists proposed that light was a wave. Then Sir Isaac Newton after observing photoelectric phenomenon, decided light was indeed made up of particles called photons.
Maxwell, who showed light was just an extension of the electromagnetic spectrum, again overturned the particle theory of light. Electromagnetism is the confluence of electricity and magnetism. As an electric current passes though a wire, it creates a magnetic field around the wire. If we vary the electric current, the magnetic field varies. A varying magnetic field induces an electric field around it. This varying electric field creates more varying magnetic fields. The effects build on each other and create what is known as an electromagnetic wave. This electromagnetic wave is self-propagating (electricity creates magnetism and vice versa) and it travels out into space at the speed of light. This convoluted theory is clearly shown by a two simple equations discovered by Maxwell and experimentally verified by a lot of eminent scientists since then.
An electromagnetic field has a frequency—the frequency is how rapidly the magnetic field (or the conjoint electric field) changes polarity. Radio waves do it anywhere from a million to a trillion times a second. Light is the same thing, but its frequency is in the hundreds of trillions range (to be more precise, 430 to 770 Gigahertz). The frequency of light, determines its color.
The wave theory again took a back seat with the advent of quantum mechanics. Again, light became a particle, or a stream of photons. All matter is composed of atoms. Atoms are composed of electrons and protons. The wispy electrons whiz around the heavy protons like planets around the sun. How high the electron floats above the proton depends on its energy of the electron. At times, the electron is quite high and decides to drop down, loosing energy and creating a photon, out of thin air. This photon shoots outwards at a horrendous speed and thus light is created. The lower energy electron then decides the grab some energy from some other source (typically heat) and float up again, just to fall down later creating another photon. Hence light is a stream of photons emanating from the electron dance. The photons have no mass and travel at the speed of light. They illuminate everything in its path, by hitting an object and bouncing off of it.
But the particle theory does not hold water either. So the modern day scientists, in their infinite wisdom, have decided not to be proven wrong again. So light is now a wave as well as a particle. Or better, the wave and particle theories of light are two simplifications of the real thing. The real thing is both, or neither, or something else, depending what you have been drinking today.
Suppose we are in a large dark room, and we light a light bulb. The light is enough for us to see, but not quite bright. Now we light another bulb. The room gets brighter, but not by a lot. If we light the third bulb, the difference is hardly noticeable, even a fourth bulb does not increase the brightness much beyond the three bulbs. So let us turn off the third and fourth bulbs, note the level of light with two bulbs, and then turn on two more bulbs. Yes, going from two to four makes a difference, just about as much as going from one to two.
The light bulb experiment shows that light perception is logarithmic. We notice brightness increases as the light doubles. So if we have 100 light bulbs lighting up a huge hall, getting it brighter would need another 100 bulbs. If that’s not enough, another 100 will not do much good, we need to add 200 more. Want it brighter? Get 400 more.
The doubling of light is the basis of exposure control in photography. When we choose a shutter speed and an aperture setting, we are essentially controlling the amount of light reaching the film. Suppose we find we need an aperture of f/2.8 and a shutter speed of 1/500 of a second. We could then make the shutter go slower and provide an exposure of 1/250 second, doubling the amount of light reaching the film. To make up for the excess light we now need to lower the aperture to f/4. Sounds confusing?
The aperture of a camera is the diameter of the hole in the lens. f/2.8 means that the hole has a diameter that is 1/2.8 of its focal length (a fixed number). In order for us to let half as much light in, we should halve the area of the hole. That is we should divide the diameter by the square root of two (about 1.4). Now we find, that 1/2.8 divided by 1.4 is 1/4.
In the present world, light plays an immensely important part of our existence. Not only does it control our visual experience and our emotions, it is ubiquitous in the technology we use. In the entertainment world, our movies and television is all about light. Tiny lasers in our CD plays provide music. The same techniques do movies from DVD disks. Software is stored optically. Digital circuits and microprocessors are imaged onto silicon wafers by light.
The world has become smaller due to widespread telephony. Tiny pulses of light traveling through thin strands of glass are the basis of fiber-optic communications. Deep under our oceans are thousands of miles of optical fiber carrying the world’s communication channels. Data, voice, video and music travel across the globe in a matter of milliseconds. The single most important driving force in the technological marvels that use light, is the laser.
The laser was once a novelty, one predicted to provide us with holography. Holography never quite delivered the goods, but the laser became the workhorse of the modern world. Laser is light in its purest form. A beam of light, with exactly one frequency and one polarization and one direction for all of the photons is a laser beam. The applications cover communications, entertainment, computing, storage, medicine, manufacturing, measuring and a whole lot more.
Light travels fast and far, but even the light stops somewhere. It stops at a back hole. A black hole is a star that is so massive that it collapses under its own weight. The matter composing a black hold gets squeezed into almost nothing in volume. This makes its gravitational force so intense that light cannot escape from it. A black hole is very hot, so it generates a lot of light, but the light never shines out. The heat does not escape either (as it is a form of light). The black hole thus cannot be seen. It seems to be yet another invention of scientists gone haywire. However, black holes have been detected, as they bend light from points beyond them. A sunny day can light up your life, but a black hole can ruin your entire day.
Partha Dasgupta is on the faculty of the Computer Science and Engineering Department at Arizona State University in Tempe. His specializations are in the areas of Operating Systems, Cryptography and Networking. His homepage is at http://cactus.eas.asu.edu/partha