Electromagnetic Waves

elma.gif (3698 bytes)


An interactive Java Applet is available for this page.  This applets shows the propagation of electromagnetic waves and demonstrates how the electric and magnetic fields are at 90° to each other.

Click here to enjoy this applet.


Previously Asked Questions

Q:    Does a current carrying wire emit an electromagnetic wave?

A:    An electromagnetic wave is emitted when an electric field, variable in time, generates a magnetic field which is also variable in time.  In its turn the magnetic field will re-generate the electric field and the process will continue indefinitely.  If the wire carries a DC current, an electromagnetic wave is emitted only during opening or closing of the circuit, when the current varies in time.  If the wire carries an AC current, an electromagnetic wave is emitted at all times since the current varies sinusoidal.

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References

Facts

The speed of an electromagnetic/light wave in a vacuum is denoted by the constant c.  c = 299792458 m/sec approx.gif (53 bytes) 3.00 x 108 m/sec

Definitions

candela:  the base unit of luminous intensity in the International System of Units that is equal to the luminous intensity in a given direction of a source which emits monochromatic radiation of frequency 540 x 1012 hertz and has a radiant intensity in that direction of 1/683 watt per unit solid angle.   Also called candle; abbreviation cd.

candlepower:  luminous intensity expressed in candelas.  Abbreviation cp.

foot-candle:  a unit of illumination on a surface that is everywhere one foot from a uniform point source of light of 1 candle and equal to one lumen per square foot.  Approximately 10.7639 lux.  Abbreviation ftc.

foot-lambert:  a unit of luminance (photometric brightness) equal to 1/pi2.gif (831 bytes) candela per square foot, or to the luminance of a perfectly diffusing surface that emits or reflects one lumen per square foot.  Approximately 3.42625 nit.  Abbreviation ft-L.  Called also equivalent foot-candle.

illuminance:  the luminous flux per unit area on an intercepting surface at any given point.  Also called illumination; luminous flux density.

lambert:  A unit of luminance (photometric brightness) that is equal to 1/pi2.gif (831 bytes) candela per square centimeter, or to the luminance of a perfectly diffusing surface that emits or reflects one lumen per square centimeter.   Abbreviation L.

lumen:  a unit of luminous flux equal to the light emitted in a unit solid angle (1 steradian) by a uniform point source of one candela intensity.  Abbreviation lm.

lumen-hour:  a unit quantity of light (luminous energy), equal to the quantity of light radiated or received for a period of 1 hour by a flux of 1 lumen.  Abbreviation lm-hr

lumen per watt:  the unit of luminosity factor and luminous efficacy.  Abbreviation lm/w.

lumen-second:  a unit quantity of light (luminous energy), equal to the quantity of light radiated or received for a period of 1 second by a flux of 1 lumen.  Abbreviation lm-sec

luminance:  the luminous intensity of a surface in a given direction per unit of projected area.  Also called brightness.

luminance factor:  The ratio of the luminance of a body when illuminated and observed under certain conditions to that of a perfect diffuser under the same conditions.

luminosity factor:  The ratio of the luminous flux in lumens emitted by a source at  a particular wavelength to the corresponding radian flux, in watts, at the same wavelength  this is a measure of the visual sensitivity of the eye.  Also called luminosity.

luminosity function:  A standard measure of the response of an eye to monochromatic light at various wavelengths  the function is normalized to unity at is maximum value. Also called luminosity curve; spectral luminous efficiency; visibility function.

luminous coefficient:  A measure of the fraction of the radian power of a light source which contributes to its luminous properties, equal to the average of the luminosity function at various wavelengths, weighted according to the spectral intensity of the source.  Also called luminous efficiency.

luminous efficacy:  1.  The ratio of the total luminous flux in lumens emitted by a source of light over all wavelengths to the total radiation flux in watts.  2.  The ratio of the total luminous flux emitted by a light source to the power input of the source; expressed in lumens per watt.

luminous emittance:  The emittance of visible radiation weighted to take into account the different response of the human eye to different wavelengths of light; in photometry, luminous emittance is always used as a property of a self-luminous source, and therefore should be distinguished from luminance.   Also called luminous excitance.

luminous energy:  The total radiant energy emitted by a source, evaluated according to its capacity to produce visual sensation.   Measured in lumen-hours or lumen-seconds.

luminous flux:  The time rate of flow of radiant energy, evaluated according to its capacity to produce visual sensations; measured in lumens.

luminous intensity:  The luminous flux incident on a small surface which lies in a specified direction from a light source and is normal to this direction, divided by the solid angle (in steradian) which the surface subtends at the source of light.  Called also light intensity.

lux:  a unit of illumination equal to the direct illumination on a surface that is everywhere one meter from a uniform point source of one candela intensity or equal to one lumen per square meter.  Abbreviation lx.   Also called meter-candle. 

nit:  a unit of luminance, equal to 1 candela per square meter.  Abbreviation nt.

phot:  A unit of illuminance equal to the illumination of a surface, 1 square centimeter in area, on which there is a luminous flux of 1 lumen, or the illumination on a surface all points of which are at a distance of 1 centimeter from a uniform point source of 1 candela.  Also called centimeter-candela.

Equations

Magnitudes of the electric field E and the magnetic field B in an  electromagnetic wave traveling along the x-axis E = Em sin (kx - omega2.gif (834 bytes)t)
B = Bm sin (kx - omega2.gif (834 bytes)t)
(where Em  and Bm are the amplitudes of E and B)
Speed of any electromagnetic wave in vacuum 34-5.gif (271 bytes)
Rate per unit area at which energy is transported by an electromagnetic wave (the Poynting vector S) 34-21.gif (205 bytes)
Intensity of an electromagnetic wave 34-26.gif (215 bytes)
Intensity of an electromagnetic wave at a distance r from a point source of power Ps 34-27.gif (176 bytes)
Radiation force if the radiation is totally absorbed F = IA/c
Radiation force if the radiation is totally reflected back along its original path F = IA/c
Radiation pressure for total absorption pr = I / c
Radiation pressure for total reflection back along path pr = 2I / c
Intensity (I) of initially unpolarized light with intensity I0 after being passed through a polarizing sheet I = onehalf.gif (67 bytes) I0
Intensity (I) of initially polarized light with intensity I0 after being passed through a polarizing sheet at angle theta2.gif (833 bytes) to the original direction of the light I = I0 cos2 theta2.gif (833 bytes)
Relation of the angle of incidence to the angle of refraction n1 sin theta2.gif (833 bytes)1 = n2 sin theta2.gif (833 bytes)2
Critical angle, if the angle of incidence exceeds the critical angle it will experience total internal reflection 34-47.gif (212 bytes)
Brewster angle, in light strikes the boundary between two mediums at the Brewster angle it will be fully polarized by reflection 34-48.gif (216 bytes)

[Top] [Previously Asked Questions] [References]


List of Topics

Measurements Electric Potential Magnetism Electrical Circuits (AC) Optical Instruments: Mirrors and Lenses
Electrostatics Capacitance Sources of Magnetic Fields Maxwell's Equations Interference
Electric Fields Current and Resistance Magnetism in Matter Electromagnetic Waves Diffraction
Electric Flux Electrical Circuits (DC) Electromagnetic Induction Interaction of Radiation with Matter: Reflection, Refraction, Polarization  

 

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