Light: an electromagnetic wave

Electromagnetic Waves It took a long time for scientiests to accept the notion that no material was needed for some types of waves. In particular, light waves are disturbances of the electromagnetic field, a nonmaterial physical entity whose quiescent state is the vacuum itself. But when one wants to visualize this field one often thinks of something similar to the stretched rope along which rope wave propagate. It is known that there are two kinds of electric charge, positive and negative, and that opposite charges attract, likes repel. A negative charge attracts a positive one, that is, the positive charge feels a force even though the two are not touching. So there is something pulling on the positive charge, even if there is nothing material immediately near it. This is called the electric field (of the negative charge in this case). Even if no positive charge is actually nearby, the negative charge still has an electric field around it, which is ready and waiting to pull or push on any charge that ventures into its region. This ready-and-waitingness can be described by electric field lines. (a) A negative charge with its field lines. The field lines show the direction of force a positive charge would feel if it were at a point on one of the lines. If there happens to be no such positive charge, the field lines are still there. (b) A spider with its web. The web is there even if there is no fly caught in it. If you wiggle a a charge up and down, you move the ends of all the field lines attached to it up and down. However, the hump that propagates along the horizontal lines will be largest, whereas a vertical field line will only slide along the direction of its length and, therefore, suffer no propagating displacement. Hence, no wave is propagated in the direction along which the charge wiggles, and the strongest waves are propagated in directions that are perpendicular to the direction of the wiggle. (a) A positive charge at the center has just moved up and down. The hump in the horizontal field lines is largest. The vertical field lines are not affeacted by this charge motion. (b) When field lines, due to both charges are added together, one gets loops of field lines. These loops propagate primarily in a horizontal direction, perpendicular to the direction of charge motion. Properties of Light Light waves are oscillations of the electric field These oscillations have frequencies --> colors The frequency is related o the wavelength The oscillations are called electromagnetic waves or electromagnetic radiation . Electromagnetic Radiation is is created through "wiggling" charges "Wiggling charges" cause oscillations of electric and magnetic field Electromagnetic waves are self-propagating waves made up of electric and magnetic fields fluctuating together. Energy is transferred not through matter, but through electric and magnetic fields. A typical electromagnetic wave consists of electric and magnetic fields arranged at right angles to each other and perpendicular to the direction the wave.gif is moving. At the point A the electric and magnetic fields associated with the wave are at maximum strength, but these fields are changing rapidly, so both decrease in strength. At point B the fields are at minimum strength and begin to increase. Thus at point A the electric and magnetic fields begin to die out, while at point B just the reverse happens. In this way the electromagnetic wave leapfrogs through space, bouncing its energy back and forth between electric and magnetic fields as it goes. The wave keeps itself going through its own internal mechanism. Electromagnetic waves are a form of radiant energy, or radiation, created when electric charges accelerate. Once they start moving, however, they do not longer depend on the source that emitted them. Speed of light c = 3 * 108 m/s Light has frequencies between 4 and 7.5 * 1014 Hz Long wavelength : infrared Medium wavelength : visible (400 - 700 nm) Short wavelength : ultraviolet n stands for nano, and 1 nm = 10-9 m The electromagnetic spectrum includes all kinds of waves that travel at the speed of light, including radio, microwave, infrared, visible light, ultraviolet, X-rays, and gamma rays. Note that sound waves, water waves, seismic waves, and other kinds of waves that require matter in order to move travel much slower than light speed. (different image of the sprectrum.) The speed of light is the highest speed possible. (Albert Einstein, Theory of Special Relativity) NOVA online: Tour the Spectrum NASA: Electromagnetic Spectrum Visible Light The relative sensitivity of the human eye differs for different wavelengths. Our perception peaks at wavelengths that we perceive as green-yellow, though the colors we see have no specific physical significance.