![]() In particular, the Dutch physicist Christian Huygens was bothered by the way that two different light beams could pass right though each other without any observable effect. (Something like a golf ball suddenly rolling into the hole.) This derivation clenched it, as far as most physicists were concerned. Newton was able to derive the laws of refraction by assuming that the particles were "kicked" by a force as they entered the glass. Color is to light what pitch is to sound.) Light refracts (changes direction) when it enters a transparent medium such as glass or water, which is what allows a lens to focus light. (In fact, since light is a wave, the different colors are due to differing frequencies. The different colors were due to different kinds of particles in his view. Newton, of course, was a great believer in the mechanical universe, so it seemed to him that light must consist of small particles, zipping through the vacuum between the Earth and Sun at high speed. Newton's experiments demonstrated that color was a property of the light itself, not of glass. Up to then, it had been thought that colors were imparted to light by the glass they passed through. He noticed that white light could be separated into colors with a prism.and then the colors could be recombined with a second prism to form white light again. Isaac Newton performed some important experiments on light during the 1670's. Light speed is 299,800,000 meters per second (186,282 miles/sec), which is far too fast to be measured by such a method. He concluded that if the speed of light was not infinite, then it must be very, very high. But the delay was so short that Galileo realized he was only observing the reaction time of his assistant. Galileo tried to measure the speed of light by standing on a hill with a lantern, and having an assistant with another lantern stand on a hill some miles away so that Galileo could observe the delay between when he opened his lantern and when he saw the return signal from his assistant. ![]() Galileo, of course, was certain that if Aristotle said it, then it must be wrong. (A billiard ball bouncing off a side rail is a good example.) This seemed to be further evidence that light moved. Also, reflected light obeys the law that the angle of incidence equals the angle of reflection, and this is the same way that hard objects behave if they meet hard walls. They thought light was a substance which moved from place to place. Many ancient thinkers disagreed with this, however, and noted the way that light can shine though clouds or windows in straight lines which seem to come from the light source. In this view, light does not have a speed (or the speed is infinite) and it doesn't even have a separate existence from the light source light just is. Aristotle thought that light was an inherent property of fire and other luminous substances. The other wave phenomena that people can sense directly is light. (The top graph shows each sound wave separately the bottom graph shows the result after they are combined algebraically.) Ground" on the other, and slowly moves from constructive interference to destructive and back again. ![]() Because they have slightly different frequencies (and therefore slightly different wavelengths) one of the waves gradually "gains This phenomena occurs when two waves of identical shape and amplitude, but slightly different frequency, interfere. I also demonstrated two tuning forks which produced clear and steady tones when played by themselves, but exhibited very strong "beats" (periodic oscillation in the volume) when played together. In other words, I could create destructive interference and show how the volume of the shriek drops noticeably. By changing the length of one of the tubes, I could delay the sound wave traveling through that tube just enough so that it arrived about one-half wavelength behind the wave in the other tube. In class, I demonstrated a "trombone" in which a quite annoying, high-pitched shriek is split and sent through two tubes before being recombined. So, we can have either constructive interference (amplitudes are added) or destructive interference (amplitudes are subtracted). ![]() Since the medium propagating the waves can only be at one point, regardless of how many waves are passing that point, the amplitudes of the waves must be combined algebraically. Interference occurs when two waves of very similar frequency and amplitude occupy the same physical space. Phyx 103-0, Light & Sound Light and Sound
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