The last quantity we will consider is the period of a wave. A wave’s period is the length of time it takes for one wavelength to pass by a given point in space. Mathematically, the period ( T
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Physics library
Course: Physics library > Unit 14
Lesson 1: Introduction to electromagnetic waves
Light: Electromagnetic waves, the electromagnetic spectrum and photons
Electromagnetic waves and the electromagnetic spectrum
Polarization of light, linear and circular
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Properties of electromagnetic radiation and photons
Electromagnetic radiation is one of the many ways that energy travels through space. The heat from a burning fire, the light from the sun, the X-rays used by your doctor, as well as the energy used to cook food in a microwave are all forms of electromagnetic radiation. While these forms of energy might seem quite different from one another, they are related in that they all exhibit wavelike properties.
If you’ve ever gone swimming in the ocean, you are already familiar with waves. Waves are simply disturbances in a particular physical medium or a field, resulting in a vibration or oscillation. The swell of a wave in the ocean, and the subsequent dip that follows, is simply a vibration or oscillation of the water at the ocean’s surface. Electromagnetic waves are similar, but they are also distinct in that they actually consist of 2
waves oscillating perpendicular to one another. One of the waves is an oscillating magnetic field; the other is an oscillating electric field. This can be visualized as follows:
Electromagnetic radiation can be drawn as an oscilating electric field (oscilating in the plane of the page/computer screen), and a perpendicular (in this case, oscillating in and out of the page) magnetic field. The Y axis is amplitude, and the X axis is distance in space.
Electromagnetic waves consist of an oscillating electric field with a perpendicular oscillating magnetic field. Image from UC Davis ChemWiki, CC-BY-NC-SA 3.0
While it’s good to have a basic understanding of what electromagnetic radiation is, most chemists are less interested in the physics behind this type of energy, and are far more interested in how these waves interact with matter. More specifically, chemists study how different forms of electromagnetic radiation interact with atoms and molecules. From these interactions, a chemist can get information about a molecule’s structure, as well as the types of chemical bonds it contains. Before we talk about that, however, it’s necessary to talk a little bit more about the physical properties of light waves.
Basic properties of waves: Amplitude, wavelength, and frequency
As you might already know, a wave has a trough (lowest point) and a crest (highest point). The vertical distance between the tip of a crest and the wave’s central axis is known as its amplitude. This is the property associated with the brightness, or intensity, of the wave. The horizontal distance between two consecutive troughs or crests is known as the wavelength of the wave. These lengths can be visualized as follows:
A two-dimensional representation of a wave. The amplitude is the distance from its central axis (indicated by the red line) to the tip of a crest. The wavelength is the distance from crest to crest, or from trough to trough.
The basic characteristics of a wave, including amplitude and wavelength. Image from UC Davis ChemWiki, CC-BY-NC-SA 3.0.
Keep in mind that some waves (including electromagnetic waves) also oscillate in space, and therefore they are oscillating at a given position as time passes. The quantity known as the wave’s frequency refers to the number of full wavelengths that pass by a given point in space every second; the SI unit for frequency is Hertz ( Hz )
, which is equivalent to “per seconds” (
. As you might imagine, wavelength and frequency are inversely proportional: that is, the shorter the wavelength, the higher the frequency, and vice versa. This relationship is given by the following equation:
c = λ ν
where λ
(the Greek lambda) is the wavelength (in meters, m
(the Greek nu) is the frequency (in Hertz, Hz
). Their product is the constant c
, the speed of light, which is equal to 3.00 × 10 8 m/s
. This relationship reflects an important fact: all electromagnetic radiation, regardless of wavelength or frequency, travels at the speed of light.
To illustrate the relationship between frequency and wavelength, let’s consider an example.
dance
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External Websites
- Case Western Reserve University – Encyclopedia of Cleveland History – Dance
- History World – History of Dance
- Stanford Encyclopedia of Philosophy – The Philosophy of Dance
Britannica Websites
Articles from Britannica Encyclopedias for elementary and high school students.
- dance – Children’s Encyclopedia (Ages 8-11)
- dance – Student Encyclopedia (Ages 11 and up)
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External Websites
- Case Western Reserve University – Encyclopedia of Cleveland History – Dance
- History World – History of Dance
- Stanford Encyclopedia of Philosophy – The Philosophy of Dance
Britannica Websites
Articles from Britannica Encyclopedias for elementary and high school students.
- dance – Children’s Encyclopedia (Ages 8-11)
- dance – Student Encyclopedia (Ages 11 and up)
Written by
Judith R. Mackrell
Dance critic, The Guardian, London.
Judith R. Mackrell
Fact-checked by
The Editors of Encyclopaedia Britannica
Encyclopaedia Britannica’s editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.
The Editors of Encyclopaedia Britannica
Last Updated: Oct 18, 2023 • Article History
Table of Contents
Pieter Bruegel the Elder: Peasant Dance
Category: Arts & Culture
Recent News
Oct. 17, 2023, 6:33 AM ET (Yahoo)
dance, the movement of the body in a rhythmic way, usually to music and within a given space, for the purpose of expressing an idea or emotion, releasing energy, or simply taking delight in the movement itself.
Dance is a powerful impulse, but the art of dance is that impulse channeled by skillful performers into something that becomes intensely expressive and that may delight spectators who feel no wish to dance themselves. These two concepts of the art of dance—dance as a powerful impulse and dance as a skillfully choreographed art practiced largely by a professional few—are the two most important connecting ideas running through any consideration of the subject. In dance, the connection between the two concepts is stronger than in some other arts, and neither can exist without the other.
Although the above broad definition covers all forms of the art, philosophers and critics throughout history have suggested different definitions of dance that have amounted to little more than descriptions of the kind of dance with which each writer was most familiar. Thus, Aristotle’s statement in the Poetics that dance is rhythmic movement whose purpose is “to represent men’s characters as well as what they do and suffer” refers to the central role that dance played in classical Greek theatre, where the chorus through its movements reenacted the themes of the drama during lyric interludes.
The English ballet master John Weaver, writing in 1721, argued on the other hand that “Dancing is an elegant, and regular movement, harmoniously composed of beautiful Attitudes, and contrasted graceful Posture of the Body, and parts thereof.” Weaver’s description reflects very clearly the kind of dignified and courtly movement that characterized the ballet of his time, with its highly formalized aesthetics and lack of forceful emotion. The 19th-century French dance historian Gaston Vuillier also emphasized the qualities of grace, harmony, and beauty, distinguishing “true” dance from the supposedly crude and spontaneous movements of early man:
Britannica Quiz
Dance
The choreographic art . . . was probably unknown to the earlier ages of humanity. Savage man, wandering in forests, devouring the quivering flesh of his spoils, can have known nothing of those rhythmic postures which reflect sweet and caressing sensations entirely alien to his moods. The nearest approach to such must have been the leaps and bounds, the incoherent gestures, by which he expressed the joys and furies of his brutal life.
John Martin, the 20th-century dance critic, almost ignored the formal aspect of dance in emphasizing its role as a physical expression of inner emotion. In doing so, he betrayed his own sympathy toward the Expressionist school of modern American dance: “At the root of all these varied manifestations of dancing . . . lies the common impulse to resort to movement to externalise states which we cannot externalise by rational means. This is basic dance.”
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A truly universal definition of dance must, therefore, return to the fundamental principle that dance is an art form or activity that utilizes the body and the range of movement of which the body is capable. Unlike the movements performed in everyday living, dance movements are not directly related to work, travel, or survival. Dance may, of course, be made up of movements associated with these activities, as in the work dances common to many cultures, and it may even accompany such activities. But even in the most practical dances, movements that make up the dance are not reducible to those of straightforward labour; rather, they involve some extra qualities such as self-expression, aesthetic pleasure, and entertainment.
This article discusses the techniques and components of dance as well as the aesthetic principles behind its appreciation as an art. Various types of dance are discussed with emphasis on their style and choreography. The history of dance in various regions is treated in a number of articles; see dance, African; music and dance, Oceanic; dance, Western; arts, Central Asian; arts, East Asian; arts, Islamic; dance, Native American; arts, South Asian; and arts, Southeast Asian. The interaction between dance and other art forms is discussed in folk dance.
