Waves

The Nature of Waves

A wave is a repeating disturbance that transfers energy as it travels through matter or space. The world around you is full of many different types of waves. Sound waves, ocean waves, light waves, and radio waves are all types of waves.

When something vibrates, it moves back and forth. The way an object vibrates demonstrates the properties of that vibration and is directly related to the properties of the wave that forms. The ripples traveling across the surface of a pond, for example, are vibrations produced when a leaf disturbs the surface of the water. The swinging motion of a giant pendulum is another example of a vibration. So, too, is the flapping of a bird’s wings and the rhythmic jolts of Earth’s crust during an earthquake.

Waves, Matter, and Energy

An important property of waves is that they transfer energy, but not matter, from place to place. For example, you can create a wave using a length of rope and a table, as shown in the diagram on the next page. Hold on to one end of the rope and then flick your wrist to send a wave moving along the rope away from you. The wave does not carry matter (the rope) along with it. Although the wave moves the rope up off the table, there is no additional rope at the other end of the rope after the wave passes.

Waves do not transfer matter from place to place - they only transfer, or transmit, energy. In the rope example, the wave, not the rope, moves away from the wrist, which is the source of the wave. The same is true for water waves. The disturbance, or wave, moves away from the source, but the water - the matter - does not. When waves splash onto a beach over and over again, the same water is oscillating to bring energy, not water, to the shore.

Waves

A wave’s movement continues as long as the wave has energy to carry. This energy has the ability to do work. When a wave encounters an object along its path, it does work on the object. For example, a resting boat will be set into motion by a passing water wave. As work is done on the boat, the wave loses energy. Waves lose energy as they do work. Sea cliffs are formed in part because energy-carrying waves erode the rock below them.

Waves transmit energy in various ways and through all phases of matter. An example of a solid transmitting wave energy is the seismic wave produced by an earthquake that takes place when rocks are under pressure and snap or slide into new positions. Waves that are felt and seen in water are examples of a liquid medium transmitting wave energy. Gases also transmit wave energy, as in an explosion, when heat and sound waves are generated.

Think about Science

Directions: Answer the following questions.

  1. In a stadium wave, a section of fans stands up with their arms raised and then sit back down. When executed in succession from one section to the next a visual wave is created that spreads around a circular stadium. Which part of a stadium wave represents the source of the disturbance? A. The stadium B. The fans in the stadium C. A fan standing up and down D. The motion of the wave around the stadium
  2. A boat is about half a kilometer offshore. A strong wind begins to blow. Which of the following best describes the transfer of energy that will take place between the wind, the boat, and the water?
    A. Energy in the wind will be transferred to the boat and the water. B. Energy in the boat will be transferred to the wind and the water. C. Energy from the water will be transferred to the wind and the boat. D. Energy from the boat will be transferred to the shoreline.

Wave Types and Their Properties

You can discriminate, or tell apart, one type of wave from another by understanding the types and properties of waves. Waves can be divided into two major categories-mechanical and electromagnetic-based on whether they require matter to travel. Waves can also be categorized based on the direction of the vibration relative to the direction of the wave velocity.

Mechanical Waves and Electromagnetic Waves

Electromagnetic waves can carry energy through matter or through space, while mechanical waves require a medium to carry energy. A medium is the matter through which waves travel. Longitudinal and transverse waves are types of mechanical waves. Although mechanical waves cannot travel through space, electromagnetic waves can. In fact, electromagnetic waves are able to travel not only through the vacuum of space but through all states of matter. Radio waves, microwaves, infrared waves, and X-rays are some of the types of electromagnetic waves that make up the electromagnetic spectrum.

Transverse Waves and Longitudinal Waves

In a transverse wave, the vibration occurs at right angles to the direction in which the wave travels. Some mechanical waves, such as water waves, are also transverse waves. In the example using the rope on the table, when flicked, the rope moved up and down, whereas the wave moved parallel to the top of the table. The two motions are at right angles to each other. The peaks, or crests, and valleys, or troughs, in the rope match the up-and-down motion of the flicking hand. If the harld moves up and down at a constant rate, the crests and troughs will be spaced at equal intervals. Like the wave in the rope, the direction of a water wave, an example of a transverse wave, is at a right angle to the movement of the water.

The second type of mechanical wave is a longitudinal, or compression, wave. In a longitudinal wave, such as a sound wave or a seismic wave, the medium moves back and forth parallel to the direction in which the wave travels. You can create a longitudinal wave by squeezing together several coils of a stretched-out spring and releasing them. You will observe that the vibration of the medium, the coil, is parallel to the wave’s motion.

Parts and Properties of Waves

All waves can be identified by their characteristics and properties. Transverse waves are characterized by a repeating pattern of crests and troughs. The rest position represents the medium when a wave is not traveling through it. In longitudinal waves, a repeating pattern of compressed and non-compressed sections is produced. The more compressed areas are areas of compression, while the less compressed areas are areas of rarefaction, which are areas of reduced pressure or density.

Wavelength

Wavelength is the distance between a point on a wave and the next identical point. For a transverse wave, this means from the top of one crest to the top of the next crest, or from the bottom of one trough to the bottom of the next trough. In a longitudinal wave, wavelength is measured from the center of one compression to the center of the next compression, or from the center of one rarefaction to the center of the next rarefaction. Waves vary in wavelengths. Ocean waves have wavelengths measured in meters (m), whereas light waves are measured in billionths of a meter (0.000000001 m), or nano-meters.

Longitudinal Waves and Transverse Waves

Frequency

The frequency of a wave is the number of wavelengths that pass a given point in one second (1 s) and is measured in units of hertz (Hz). One hertz is equal to one complete cycle, or wavelength, per second. Thus, if 5 wavelengths pass each second, the wave has a frequency of 5 cycles/s, which is expressed as 5 Hz. The frequency of waves can vary widely. For example, the frequency of ocean waves is less than 1 Hz, while the frequency of visible light is about 1,000,000,000,000,000 Hz (1015 Hz).

Speed

If a wave’s frequency and wavelength are known, the wave’s speed can be calculated by using the following equation:

This equation can also be written using the symbol for speed, the symbol for frequency, and the Greek letter lambda for wavelength, or:

The speed of a wave is dependent upon the medium through which it travels. Sound waves, for example, travel at approximately 340 mis through air, at more than 1,400 mis in water, and at an estimated 5,800 mis in steel. Light waves are also affected by the medium through which they travel. In the vacuum of space, the speed of light is 3.0 x 108 m/s. The speed slows in gases, and it slows more in liquids and solids. Temperature also affects wave speed. The speed of sound in air is increased at higher air temperatures.

Amplitude

The amplitude of a wave is related to the amount of energy it carries. The greater the energy a wave has, the greater its amplitude. The amplitude of any transverse wave is the distance from the rest position to either a crest or a trough. In longitudinal waves, the amplitude depends on how tightly the medium is squeezed together in its regions of compression. Using a coil as a model, imagine areas of a wave that are squeezed together. When compressions are more tightly squeezed, the wave has a higher amplitude.

Think about Science

Directions: Answer the following questions.

  1. Based on the formula , which of the following is true for a sound wave whose velocity is 340 m/s? A. As the frequency of a wave increases, its wavelength decreases. B. As the frequency of a wave increases, its wavelength increases. C. As the frequency of a wave increases, its wavelength stays the same. D. As the frequency of a wave increases, its wavelength increases twice as quickly.

The Electromagnetic Spectrum

All waves transmit energy as they travel. The transfer of energy along electromagnetic waves is called radiation. The complete range of electromagnetic waves makes up the electromagnetic spectrum. The spectrum displays a continuous range of electromagnetic waves arranged by increasing frequency and decreasing wavelength. Waves with the highest frequencies have the highest energy levels. The types of waves are indicated within certain ranges of the spectrum. However, the names and frequency ranges are not exact, because they overlap in places.

Electromagnetic Spectrum

Radio Waves and Microwaves

Like all electromagnetic waves, radio waves are produced when electric charges vibrate. Radio waves are the longest in the spectrum with wavelengths that range from the length of a football to longer than a football field. Radio waves also have the lowest frequencies and carry the least amount of energy of all electromagnetic waves. Televisions radios and cellular phones use radio waves. Radio waves with the high’e st energy are microwaves. Microwave ovens and cellular phones use these higher-frequency radio waves.

Infrared Waves

The warm sensation you feel when sunlight is absorbed by your skin is the result of electromagnetic waves called infrared waves. The wavelength of infrared waves is shorter than that of radio waves but longer than that of visible light. Infrared, or IR, waves are approximately 0.00075 mm to 1 mm in length. Television remote controls, orbiting satellites, and even some animals use infrared wavelengths.

Visible Light

Visible light is the part of the electromagnetic spectrum humans can see. These visible rays of the spectrum are recognized by the human eye as color. In order of long wavelength to short wavelength, the colors are red, orange, yellow, green, blue, indigo (deep blue), and violet. When all the colors of the visible spectrum are present, the light appears white. Nearly all things are visible because they reflect light given off by the Sun or another source, such as a light bulb. Visible light is extremely important to Earth’s organisms because plants use the red and blue bands of the spectrum to create food in a process called photosynthesis.

Two theories about the nature of light focus on different properties of light. According to the Wave Theory of Light, light is a luminous energy emitted by a light source and travels through space as a transverse wave. According to the Particle Theory of Light, light energy is both radiated (transmitted) and absorbed as tiny packets, or bundles, and not as continuous waves.

Ultraviolet Waves

Ultraviolet, or UV, waves have higher frequencies than visible light waves. Wavelengths for UV light range between 10 billionths and 400 billionths of a meter long. The waves damage living cells and cause sunburn, and too much exposure to UV waves can lead to skin cancer. Ultraviolet waves, along with all other electromagnetic waves, are emitted by the Sun.

Short exposures to ultraviolet waves have several beneficial effects. The human body uses the ultraviolet energy to produce vitamin D, a necessary nutrient. Hospitals use UV waves to disinfect surgical equipment. Some materials fluoresce, or emit visible light, when struck by ultraviolet waves. Police detectives sometimes use fluorescent powder and an ultraviolet light source to look for fingerprints.

X-Rays and Gamma Rays

X-rays are high-energy electromagnetic waves with wavelengths shorter than those of ultraviolet waves. Gamma rays have the shortest wavelength and highest energy of the electromagnetic waves. Whereas ultraviolet waves can penetrate the top layer of a person’s skin, X-rays have enough energy to pass through skin and muscle. X-rays and gamma rays have important uses in medicine. Images are formed when X-rays are beamed through a person’s body. The rays strike a film plate or an electronic sensor, creating an image of internal structures such as bones. Gamma rays are used to treat some cancers. Exposing a cancerous tumor to a highly focused beam of gamma rays can kill the cancerous cells.

Think about Science

Directions: Select the correct answer to the following questions.

  1. Which statement accurately describes the organization of waves on the electromagnetic spectrum in terms of frequency, velocity, and wavelength? A. Frequency and velocity increase; wavelength decreases. B. Frequency increases; velocity and wavelength decrease. C. Frequency decreases; wavelength decreases. D. Frequency decreases; wavelength increases.
  2. What is the disturbance that produces an electromagnetic wave? A. Nuclear explosions B. Heat C. Wind D. Vibrating electrons