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# Wave Mysteries Help

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By — McGraw-Hill Professional
Updated on Sep 12, 2011

## Introduction

The more we delve into the mysteries of wave phenomena, the less we seem to know about them. Studying waves is bound to produce more questions than answers.

## Lengthwise Versus Sideways

When waves travel through matter, the molecules oscillate to and fro, up and down, or back and forth. The nature of the particle movement differs from the nature of the wave as it travels. The atoms or molecules rarely move more than a few meters—sometimes less than a centimeter—but the wave can travel thousands of kilometers. Sometimes the particles vibrate in line with the direction of wave travel; this is a compression wave , also called a longitudinal wave . In other instances, the particles move at right angles to the direction of propagation; this is a transverse wave . The distinction is illustrated in Fig. 17-8.

Fig. 17-8 . In a longitudinal wave (a) , the particles vibrate parallel to the direction of wave travel. In a transverse wave (b) , the particles vibrate laterally.

What is it that wags or wiggles or compresses or stretches when a wave travels through a particular medium? It depends on the medium and on the nature of the wave disturbance. Sound waves in air are longitudinal, but radio waves are transverse. The waves on the surface of the ocean are transverse, but when a big wave arrives on a beach, plenty of longitudinal motion is involved as well.

## Force Fields

When waves travel through a vacuum, they manifest as force fields (in the case of EM waves) or as undulations in space-time (gravitational waves). It took scientists a long time to accept the fact that waves can propagate without any apparent medium to carry them.

Both EM and gravitational waves are transverse disturbances. The electrical and magnetic force fields in radio waves, IR, visible light, UV, x-rays, or gamma rays pulsate at right angles to each other and at right angles to the direction of propagation. This occurs in three dimensions, so it can be envisioned in the “mind’s eye.” Gravitational waves are more esoteric; they cause space-time to oscillate in four dimensions. If anyone claims to be able to envision four-dimensional wave oscillation directly, you are justified in suspecting that they’re either joking or crazy. Nevertheless, they can be defined quite easily with mathematics.

## Corpuscles Of Light

The theory of EM-wave propagation is a relatively recent addition to the storehouse of physics knowledge. Isaac Newton, the seventeenth-century English physicist and mathematician known for his theory of gravitation and his role in the invention of calculus, believed that visible light consists of submicroscopic particles. To the casual observer, visible light travels in straight lines through air or free space. Shadows are cast in such a way as to suggest that there are no exceptions to this rule, at least in a vacuum. Today scientists know that light behaves, in some ways, like a barrage of bullets. Particles of EM energy, called photons , have momentum, and they exert measurable pressure on objects they strike. The energy in a beam of light can be broken down into packets of a certain minimum size but no smaller.

However, one need not search hard to find complications with Newton’s corpuscular theory of the nature of light. At a boundary between air and water, photons do inexplicable things. Ask any child who has ever stuck a fishing pole into a lake or who has looked into the deep end of a swimming pool and seen 4 m of water look like 1 m. Photons change direction abruptly when they pass at a sharp angle from water to air or vice versa (Fig. 17-9), but there is no apparent force to give them a sideways push. When light passes through a glass prism, things get stranger still; not only are light beams bent by the glass, but the extent to which they are bent depends on the color!

Fig. 17-9 . If light rays consist of particles, what pushes them sideways at the water surface?

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