Particles and waves historical essays in the philosophy of science

Rayleigh waves in an elastic solid are different from surface waves in water in a very important way. In a water wave all particles travel in clockwise circles. However, in a Rayleigh surface wave, particles at the surface trace out a counter-clockwise ellipse, while particles at a depth of more than 1/5th of a wavelength trace out clockwise ellispes. This motion is often referred to as being "retrograde" since at the surface, the horizontal component of the particle motion is in the opposite direction as the wave propagation direction. I have identified two particles in orange in this animation to illustrate the retrograde elliptical path at the surface and the reversal in the direction of motion as a function of depth.

The above diagram can be somewhat misleading if you are not careful. The representation of sound by a sine wave is merely an attempt to illustrate the sinusoidal nature of the pressure-time fluctuations. Do not conclude that sound is a transverse wave that has crests and troughs. Sound waves traveling through air are indeed longitudinal waves with compressions and rarefactions. As sound passes through air (or any fluid medium), the particles of air do not vibrate in a transverse manner. Do not be misled - sound waves traveling through air are longitudinal waves.

Today, physicists accept the dual nature of light. In this modern view, they define light as a collection of one or more photons propagating through space as electromagnetic waves. This definition, which combines light's wave and particle nature, makes it possible to rethink Thomas Young's double-slit experiment in this way: Light travels away from a source as an electromagnetic wave. When it encounters the slits, it passes through and divides into two wave fronts. These wave fronts overlap and approach the screen. At the moment of impact, however, the entire wave field disappears and a photon appears. Quantum physicists often describe this by saying the spread-out wave "collapses" into a small point.

WARNING: For most waves thought about in physics, like sound waves or light waves, when you change the frequency, you also change the wavelength. That makes the velocity constant, which is true for sound and light waves. The above animation is not meant to show what happens specifically with sound and light waves. The animation is about waves in general, and what is meant by frequency. It is meant to show what happens when only the frequency changes, and not show the corresponding change in wavelength that happens with sound and light waves.

Particles and waves historical essays in the philosophy of science

particles and waves historical essays in the philosophy of science

WARNING: For most waves thought about in physics, like sound waves or light waves, when you change the frequency, you also change the wavelength. That makes the velocity constant, which is true for sound and light waves. The above animation is not meant to show what happens specifically with sound and light waves. The animation is about waves in general, and what is meant by frequency. It is meant to show what happens when only the frequency changes, and not show the corresponding change in wavelength that happens with sound and light waves.

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particles and waves historical essays in the philosophy of scienceparticles and waves historical essays in the philosophy of scienceparticles and waves historical essays in the philosophy of scienceparticles and waves historical essays in the philosophy of science