Waves: Overview [Includes Video]
When we think of waves, we usually think of the kind we see in the ocean. But waves are all around us. Invisible waves in the air carry sound, heat, and other forms of energy. A wave can be defined as a disturbance that transmits energy from one place to another.
There are two main types of waves: mechanical and electromagnetic. Mechanical waves move through a physical substance, or medium, such as air or water. When you pluck a guitar string, for example, you create a mechanical wave as the string vibrates back and forth. This vibration disturbs the air molecules around the guitar. Eventually, this disturbance causes your eardrum to vibrate. These vibrations are then converted to an electrical signal that travels to your brain, and your brain interprets it as sound.
Electromagnetic waves are different from mechanical waves in that they move through an electric and magnetic field. They can move through a physical medium, such as water, but they can also move without a medium, making them able to travel through the emptiness of space. The light that travels to Earth from the Sun is an example of an electromagnetic wave. Heat from the Sun also travels through electromagnetic waves. The only two types of electromagnetic waves that can be sensed by people are visible light and heat; all other types are invisible. Instead, technology is used to detect these invisible waves and convert them into information.
Features of Transverse and Longitudinal Waves
Waves travel in two basic forms: transverse and longitudinal. Transverse waves move up and down along an S-shape Page 98 | Top of Articlecurve. The vibrations of a guitar string are an example of a transverse wave. Before you pluck a guitar string, it is at equilibrium. After you pluck the string, it moves back and forth across this equilibrium position. The farthest it moves from equilibrium in the positive direction is called the wave's crest. The farthest it moves from equilibrium in the negative direction is its trough. The length between each crest or trough is known as the wavelength.
Longitudinal waves follow a coil shape. An example of a longitudinal wave is sound traveling through air. Unlike transverse waves, longitudinal waves do not have crests and troughs. Instead, they have compressions and rarefactions. Compressions occur when the coils are closer together, creating higher pressure. Rarefactions occur when the coils are farther apart, creating lower pressure. The wavelength of a longitudinal wave is the distance between two compressions in a row or two rarefactions in a row.
Frequency and Amplitude
Scientists measure waves using frequency and amplitude. The frequency is how many wavelengths occur in a given period. A low-frequency wave might have only one or two Page 99 | Top of Articlewavelengths per second. A high- frequency wave might have trillions of wavelengths per second. When the period is one second, the frequency is measured in units called hertz (Hz). For example, if a sound wave cycles through five wavelengths per second, its frequency is 5 Hz.
When you pluck a guitar string, the pitch of the sound you hear depends on the frequency of the wave that is created. Page 100 | Top of ArticleThe higher the note you play, the faster the string vibrates. Higher notes create shorter, more rapid waves. Your brain interprets these higher-frequency waves as having a higher pitch. Lower notes vibrate more slowly, creating longer waves. Your brain interprets these lower-frequency waves as having a lower pitch.
Amplitude is a measure of the amount of energy a wave carries. In the example of the guitar, when you pluck a string with great force, the wave it creates is taller, or has a higher amplitude. Your brain interprets high-amplitude sound waves as being loud. If you pluck the string softly, the wave it creates is shorter, or has a lower amplitude. Your brain will interpret these low-amplitude sound waves as being soft.
Much of the technology we use today relies on electromagnetic waves, or electromagnetic radiation. Televisions, Wi-Fi, Bluetooth, and microwave ovens all rely on electromagnetic waves to work. Scientists classify electromagnetic waves according to their frequency. When we put all of the electromagnetic wave frequencies in order from lowest to highest, it is called the electromagnetic spectrum.
Radio waves and microwaves are on the lower end of the spectrum. They are examples of lower-frequency electromagnetic waves. Light falls roughly in the middle of the spectrum. This includes both the light that humans can detect with their eyes and the light that they cannot. Infrared is light whose frequency falls below the threshold of human sight, while ultraviolet is light whose frequencies lie above the range of human sight. Very high-frequency waves are found at the top of the spectrum. These include X-rays and gamma rays. Page 101 | Top of ArticleSuch waves are so small that they can travel through the human body. This makes them a health risk if we do not control them carefully.
Because electromagnetic waves can transmit energy without transmitting mass, they are useful for sending information. Electromagnetic waves can be used to send data through wires (for example, as light rays through fiber-optic cables). They can also be used to send information through the air, as with television and cellular telephone signals. However, humans cannot sense these waves and interpret them. Instead, we need technology to intercept and decode these waves. Wireless devices such as smartphones and radios use antennas to detect waves traveling through the air. These devices then convert the electromagnetic waves into sounds, images, and other forms of information that we can use.
Many of the devices we use in our day-to-day lives use electromagnetic waves on the lower half of the electromagnetic spectrum. But scientists and technicians have found ways to use the waves on the higher end of the spectrum too. X-rays are an example of a very-high-frequency wave. They are so small that they can pass through skin and muscle but not bone. This means that doctors can pass X-rays through our body to take a photograph of our skeleton. Any rays that hit bone are reflected onto a special photographic film. Once this film is developed, we have a snapshot of the skeleton. However, because X-rays are high-energy waves, they can cause damage to the body if we are exposed for too long. This is why X-ray technicians, who work with these high-energy waves on a daily basis, wear protective vests or aprons. These protective clothes are made from a material such as lead that can reflect X-ray radiation away from the body.