WHAT IS A WAVE?
BY CLAUDIA BRUNDU, CHIARA FLAVIANO AND SAMANTHA RAGAZZINI.
So waves are everywhere. But what makes a wave a wave? What characteristics, properties, or behaviours are shared by the phenomena the we typically characterize as being a wave? How can waves be described in a manner that allows us to understand their basic nature and qualities?
A wave can be described as a disturbance that travels through a medium from one location to another location. Consider a slinky wave as an example of a wave. When the slinky is stretched from end to end and is held at rest, it assumes a natural position known as the equilibrium or rest position. The coils of the slinky naturally assume this position, spaced equally far apart. To introduce a wave into the slinky, the first particle is displaced or moved from its equilibrium or rest position. The particle might be moved upwards or downwards, forwards or backwards; but once moved, it is returned to its original equilibrium or rest position. The act of moving the first coil of the slinky in a given direction and then returning it to its equilibrium position creates a disturbance in the slinky.
We can then observe this disturbance moving through the slinky from one end to the other. If the first coil of the slinky is given a single back and forth vibration, then we call the observed motion of the disturbance through the slinky a slinky pulse. A pulse is a single disturbance moving through a medium from one location to another location. However, if the first coil of the slinky is continuously and periodically vibrated in a back and forth manner, we would observe a repeating disturbance moving within the slinky that endures over some prolonged period of time. The repeating and periodic disturbance that moves through a medium from one location to another is referred to as a wave.
DIFFERENT TYPES OF WAVES
Transverse Waves
This type of 'up and down' wave, which looks like a water wave, is called a transverse wave. It's called this because the motion of the particles in the wave medium is perpendicular to the wave's direction. Let's say for a moment that this IS a water wave. The wave moves in this direction, like a beach wave moving toward the shore. But the particles in the medium - that is, the molecules of water - are NOT traveling toward the shore. They're simply oscillating up and down with each successive wave. Now compare the direction of the water molecules' movement to the direction the wave is traveling. What do you see? The two directions are perpendicular to each other. That's what makes this a transverse wave.
Longitudinal Waves
Longitudinal waves are a bit challenging to imagine, so let's use a slinky for reference. Imagine a long slinky is stretched out across a tabletop, with one end fixed. Imagine if you were holding the opposite end, and you applied a quick push-pull motion to the slinky. The push would cause a compression to form, which would travel down the slinky to the opposite end. This is what a longitudinal wave looks like. The motion of the oscillating medium - in this case, the slinky - is parallel to the motion of the wave itself. Longitudinal waves don't have crests and troughs like transverse waves. But they do exhibit features that can be treated as the same thing.
Transverse and Longitudinal Waves
Gravitational Waves
A gravity wave (or gravitational wave) is a ripple in the curvature of the space-timecontinuum (the enmeshed combination of our three perceived physical dimensions, plus time) created by the movement of matter. Long thought to exist before they were detected, gravity waves were first hypothesized in Albert Einstein's general theory of relativity, which predicted that an accelerating mass would radiate gravitational waves as it lost energy. For example, it would be expected that two pulsars (celestial bodies that emit radiant in regular pulses) in orbit around each other should emanate gravity waves as their orbits decay. According to theory, gravity waves propagate at approximately the speed of light and pass through matter unchanged, alternately stretching and shrinking distances on an infinitesimal scale.
Gravitational Waves
Electromagnetic Waves
Electricity can be static, like the energy that can make your hair stand on end. Magnetism can also be static, as it is in a refrigerator magnet. A changing magnetic field will induce a changing electric field and vice-versa—the two are linked. These changing fields form electromagnetic waves. Electromagnetic waves differ from mechanical waves in that they do NOT require a medium to propagate. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space. In the 1860's and 1870's, a Scottish scientist named James Clerk Maxwell summarized this relationship between electricity and magnetism into what are now referred to as "Maxwell's Equations."
Mechanical Waves
A mechanical wave is a wave that is an oscillation of matter, and therefore transfers energy through a medium. While waves can move over long distances, the movement of the medium of transmission—the material—is limited. Therefore, oscillating material does not move far from its initial equilibrium position. Mechanical waves transport energy. This energy propagates in the same direction as the wave. Any kind of wave (mechanical or electromagnetic) has a certain energy. Mechanical waves can be produced only in media which possess elasticity and inertia. A mechanical wave requires an initial energy input. Once this initial energy is added, the wave travels through the medium until all its energy is transferred.
Energy Transport and the Amplitude of a Wave
A wave is an energy transport phenomenon that transports energy along a medium without transporting matter. A pulse or a wave is introduced into a slinky when a person holds the first coil and gives it a back-and-forth motion.
This creates a disturbance within the medium; this disturbance subsequently travels from coil to coil, transporting energy as it moves. The energy is imparted to the medium by the person as he/she does work upon the first coil to give it kinetic energy. This energy is transferred from coil to coil until it arrives at the end of the slinky. If you were holding the opposite end of the slinky, then you would feel the energy as it reaches your end. In fact, a high energy pulse would likely do some rather noticeable work upon your hand upon reaching the end of the medium; the last coil of the medium would displace your hand in the same direction of motion of the coil. For the same reasons, a high energy ocean wave can do considerable damage to the rocks and piers along the shoreline when it crashes upon it.
How is the Energy Transported related to the Amplitude?
The amount of energy carried by a wave is related to the amplitude of the wave. A high energy wave is characterized by a high amplitude; a low energy wave is characterized by a low amplitude. The amplitude of a wave refers to the maximum amount of displacement of a particle on the medium from its rest position. Putting a lot of energy into a transverse pulse will not effect the wavelength, the frequency or the speed of the pulse. The energy imparted to a pulse will only affect the amplitude of that pulse.
The Speed of a Wave
The speed of an object refers to how fast an object is moving and is usually expressed as the distance traveled per time of travel. In the case of a wave, the speed is the distance traveled by a given point on the wave (such as a crest) in a given interval of time.
Reflection, Refraction and Diffraction
Reflection involves a change in direction of waves when they bounce off a barrier; refraction of waves involves a change in the direction of waves as they pass from one medium to another; and diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path. Reflection, refraction and diffraction are all boundary behaviors of waves associated with the bending of the path of a wave.
Interference of Waves
Wave interference is the phenomenon that occurs when two waves meet while traveling along the same medium. The interference of waves causes the medium to take on a shape that results from the net effect of the two individual waves upon the particles of the medium.
The Doppler Effect
The Doppler effect is observed whenever the source of waves is moving with respect to an observer. The Doppler effect can be described as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding. It is important to note that the effect does not result because of an actual change in the frequency of the source. The Doppler effect can be observed for any type of wave - water wave, sound wave, light wave, etc.
Sound Waves
Sound is a wave that is created by vibrating objects and propagated through a medium from one location to another. Sound is a mechanical wave that results from the back and forth vibration of the particles of the medium through which the sound wave is moving. If a sound wave is moving from left to right through air, then particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it. The motion of the particles is parallel (and anti-parallel) to the direction of the energy transport. This is what characterizes sound waves in air as longitudinal waves.
THE END