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Wave Length

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-193

2025-11-16

710

One particle of a body transmitting waves is in the same phase as another particle when it is moving in the same direction with the same time. Figure 1 E shows the form of a wave, due to trans verse vibrations, from left to moving right. The particle A is moving downward with a certain velocity, and the next particle that is in the same phase is E. The particle C has the same velocity, but is moving upward. The distance between any particle and the next particle in the same phase, measured in the direction of wave motion, as AE, is the wave length. The top of the wave at B is called the crest, while the bottom at D is the trough. The vertical distance from B to the horizontal line (that is, half the vertical distance between B and D) is the amplitude. It is a simple matter to make a body record its own vibrations by tracing a wave form similar to Fig. 1.

Demonstration. - Bore a hole near the end of a long piece of whalebone, and fasten it by a screw to the side of a block screwed to a board. Rub a few drops of kerosene or cosmolene on one side of a strip of glass so that the surface is evenly covered with a thin layer. Put some flour in a muslin bag and dust it evenly over the glass. Place on the board between two guides and underneath the whalebone. Fix a bristle to the whalebone near the end so that it will just touch the glass. Vibrate the whalebone, and the bristle will make in the flour surface a nearly straight line twice the length of the amplitude. Vibrate the whalebone again, and raise one end of the board so that the glass will slide out, and the vibrations will trace a beautiful wave form, as shown in Fig. 2. Since the motion of the glass strip is changing, the wave form is variable. If the strip is drawn out with uniform velocity, the wave form will closely approximate that shown in Fig. 1.