Tuesday, April 16, 2013

History of Science -- Part Five: Light

Newton discovered many physical properties of light. Newton bought his first prism in an attempt to disprove French Philosopher Rene Descartes’ wave theory of light. He argued that the geometric nature of the laws of reflection and refraction could only be explained if light was made of particles, which he referred to as corpuscles, as waves do not tend to travel in straight lines. After joining the Royal Society of London in 1672, Newton stated that the 44th trial in a series of experiments he had conducted earlier that year had proven that light is made of particles and not waves.

Advocates of the wave theory had previously stated that light waves are made of white light and that the spectrum that can be seen through a prism is formed because of corruption within the glass. This means that the more glass the light travels through, the more corrupt it will become.

In order to prove that this was false, Newton passed a beam of white light through two prisms which were held at such an angle that it split into a spectrum when passing through the first prism and was recomposed, back into white light, by the second. This showed that the spectrum was not caused by glass corrupting the light. Newton claimed that this was a “crucial experiment.”

A crucial experiment is any experiment devised to decide between two contradictory theories, whereby the failure of one determines the certainty of the other. Since almost everyone agreed that light must either be composed of particles or waves, Newton used the failure of the wave theory to prove that light is made of particles. (Ironically, this experiment did not prove a thing. In fact, if we consider the colors of light to be different frequencies of waves, then the spectrum is a better demonstration of a wave property of light.)

Newton concluded that light is composed of colored particles, which combine to appear white. He introduced the term “color spectrum” and, although the spectrum appears continuous, with no distinct boundaries between the colors, he chose to divide it into seven: red, orange, yellow, green, blue, indigo, and violet.

Newton chose the number seven as this reflected the Ancient Greek belief that it is a mystical number because there are seven “wandering stars” (the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn) and seven days in a week. Recall he was a bit of a mystic. A little bit of science, a little bit of magic, a little lucky guessing, that has been the path science has followed even up to the present day.

Newton showed that every color has a unique angle of refraction that can be calculated using a suitable prism. He saw that all objects appear to be the same color as the beam of colored light that illuminates them and that a beam of colored light will stay the same color no matter how many times it is reflected or refracted. This led him to conclude that color is a property of the light that reflects from objects, not a property of objects themselves. On this he was most correct.

Despite Newton's confidence that his theory had been proven, it still faced several problems and was not accepted right away. Within a year of his announcement fellow Royal Society member Robert Hooke argued that diffraction is not a new type of refraction as Newton had claimed and that it could only be explained by assuming that light is a wave.

Many other members joined Hooke in criticizing Newton's particle theory. Some denied that the Newton's color spectrum existed at all and others denied that his 44th trial really proved that light is not composed of waves. Those that tried to replicate Newton's experiment often failed. Prisms were not commonly accepted as scientific instruments at that time. They were sold as simple forms of entertainment and there was little technical work on their design or improvement. Venetian glass was regarded as the standard against which other glasses were compared, but even this was full of air bubbles and flaws.

Newton did not help matters by concealing the details of his trials. He did not explain how to produce a spectrum from the first prism or specify the size or geometry of the second. Further, he used glass made in London that was superior to the Italian glass prisms. However, with his usual disdain for conflict, he bowed out of the discussion.

Newton had good arguments for his conclusion that light was as stream of tiny particles. Objects obeying his universal equations of motion, light travels in straight lines unless it encounters something that might exert a force on it. His particles of light scatter widely when they hit something irregular, but bounce off a mirror at the same angle that they hit, as would a tennis ball. Newton even ascribes color to the sizes of these bodies.

Actually, Newton was conflicted. He investigated a property of light we now call “interference,” a phenomenon uniquely characteristic of waves. Nevertheless, he came down strongly in favor of particles. Waves seemed to require a medium in which to propagate, and this medium would impede the motion of the planets that his universal equation of motion seemed to deny. This problem of “just what is waving” if light is a wave is a fundamental issue that we will hear of again.

Other scientists proposed wave theories of light that also matched Newton’s experiments of reflection and refraction, but the overwhelming authority of Newton meant that his “corpuscular theory,” that light is a hail of little bodes, dominated for more than 100 years. Ironically, the “Newtonians” were more certain of Newton’s corpuscle than was Newton — until about 1800.

Thomas Young was a precocious child who, reportedly, read fluently at the age of two. He was educated in medicine, earned his living as a physician, and was an outstanding translator of hieroglyphics. But his main interest was physics. At the beginning of the nineteenth century, Young provided the convincing demonstration that light was a wave.

On glass coated with soot, Young scribed two closely spaced parallel lines. Light shining through these two slits onto a wall or reflecting screen produced a pattern of bright and dark bands we call an “interference pattern.” Interference is the conclusive demonstration of wave behavior. Not only does this experiment demonstrate the wave property of light, but it will be integral in our story later as scientists explore the properties of waves and matter. This is an important result, so lets spend some time exploring it.

You can picture a “wave” as a moving series of peaks and valleys, or crests and troughs … like an ocean wave. Such crests and troughs can, for example, be seen through the flat side of an aquarium as ripples on the water surface. Another way to depict waves is a bird’s eye view where you can draw lines to indicate the crests. Waves on the ocean seen from an airplane look like this. I’ll draw some diagrams using that method to demonstrate the principles of wave propagation and interference patterns.

Waves from a small source such as a pebble dropped into water spread out in all directions. Similarly, light from a tiny glowing object spreads in all directions. Light from a large or a distant source will appear more as a row of parallel lines. However, when waves like that encounter a small slit, it will spread out from the slit like from a point source.

Light coming through two closely spaced slits might be expected to illuminate the screen twice as brightly as the light from a single slit. That is the behavior we would expect if light were a series of tiny particles like Newton supposed. But what Young saw when he shined a light through his experimental slits was bands of bright and dark on the screen — a stream of particles could not account for this.

The explanation of the interference pattern is that light from each slit travels a different distance to the areas across the screen. At some points the crests from the light through one slit combine with the crests from the light passing through the other slit. That creates an area of brightness. However, on other parts of the screen, the crest of one wave combines with the trough of the other wave and the light “cancels out” just like adding +5 and -5. This cancelation produces the dark bands. This is the unique wave behavior of light combining and canceling out.

Since one wave is the same as the next, this effect is repeated across the screen of adding and canceling producing bands of bright and dark. “Interference” is actually a misnomer. Waves from the two slits are not interfering with each other, they just add and subtract, like deposits and withdrawals from a bank account.

If you think about the geometry a bit, you can see that the greater the spacing between the slits, the smaller the spacing between the bright bands of the interference pattern. To be truly noticeable, this effect is best produced by two slits very close together. That is what was missing in earlier experiments of this type.

But the essential point to remember is that the interference pattern shows that the light waves reaching each point on the screen must come from both slits. Were light a stream of particles, there would be no interference pattern. Little bullets, each coming through one slit or the other, could not cancel each other out to produce the pattern depending on the slit separation.

Is Young’s argument airtight? Probably not. When Young presented it, it was hotly disputed. Young’s English colleagues were strong in the Newtonian particle school of thought. Plus, wave ideas were favored by the French, and that alone was a good enough reason for a loyal Britain to reject the theory. You see, science is not immune to the prejudices that infect the human race. However, before long, further experiments overwhelmed the objections to wave theory and it was adopted as the true explanation of the form of light.

This isn’t the end of the story. This double slit experiment and its results will appear again as we explore the path of modern science. But, for now, we will follow the trail of electromagnetic force, which is a wave theory. We also need to address the progress science made in studying “heat” and “energy,” a study that had very practical applications in the age of steam engines.

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