What would happen if light emitted light?
Big Bang 7, textbook
Wave and Particle 33 RG 7.2 G 7.2 Competence Area Quantum Physics 55 The theory of light, i.e. optics, is one of the oldest areas of physics. Even the ancient Greeks discussed whether light emanates from things or whether our eyes virtually feel things. It was only around the year 1000 that the Arab natural scientist I BN A L H AITHAM seems to have made it clear that visible things emit light, that is, they shine themselves or reflect foreign light. That sounds very logical! According to the "sampling hypothesis" you could also see in the dark (F5)! But what is light? What is being sent out? In the 17th century there were two views. C HRISTIAN H UYGENS (see chap. 33.1, p. 53) was of the opinion that light is a wave. So he represented the wave theory. I SAAC N EWTON, on the other hand, was of the opinion that light was a stream of particles and thus represented the particle theory. Newton's authority was so great that his view prevailed for more than a century. But then came the year 1801, and T HOMAS Y OUNG presented an experiment with which he could clearly show that light has wave properties. The double-slit experiment he invented is an absolute classic and is still being carried out today (see, for example, Fig. 33.23, p. 60). Why was this experiment a strong clue for the wave theory of light? In order to understand the significance of the experiment, one must first consider what the light is doing behind a double slit - depending on whether it behaves like a particle or a wave. Let us first assume that light behaves like a shower of particles. On a screen behind the double slit, there should then be two bright stripes, one behind each slit. R ICHARD F EYNMAN liked to use the comparison with a machine gun with which one shoots through two slits (F7; Fig. 33.8). Fig. 33.8: If you shoot a machine gun through a “double slit”, you will get two stripes behind it with the points of impact of the bullets. But what would happen if light behaved like a wave? For all of the following considerations, we assume that light consists of only one frequency, i.e. is monochromatic. There would then be a circular wave behind each of the two gaps (Fig. 33.9). Overlaying them would lead to interference, so that the light is amplified in some places and extinguished completely in others. If light were a wave, one would have to see not just two but many bright stripes behind the double slit (F6; Fig. 33.10). And that is exactly what Y OUNG was able to show with his double slit experiment: lots of light stripes! Fig. 33.9: If you cover one of the columns, you get the familiar pattern of a circular wave (see also Fig. 33.5 a, p. 54). Fig. 33.10: The superposition of the two circular waves leads to interference. With a light wave you would then get several light stripes on the screen in this case. The experiment is viewed from above, but the screen is rotated for a better overview. Particle theory could not explain the outcome of the double slit experiment. Because two particles would not have to add to any one in the dark places, quasi 1 + 1 = 0! In the context of wave theory, however, the explanation with the help of destructive interference is not a problem. Young's experiment gradually led to a rethink. The wave theory finally prevailed and the particle theory was rejected again. Why did it take so long to realize that light has wave properties? This was known from sound much earlier. This is because the wavelength of sound is around a million times greater than that of light. The typical light wave properties therefore do not play such an obvious role in everyday life as they do with sound. Info: Shimmering CDs -> p. 56 Info: Look around the corner -> p. 56 For testing purposes only - property of the publisher öbv
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