The Symmetry of the Two Kinds of Fields Think about it: If you’re sitting at a point in space, and a charge is moving near you, the strength of its electrical field is changing. And you feel a magnetic field. In other words, a changing electrical field causes a magnetic field. But the opposite is true too: A changing magnetic field causes an electrical field. What will happen if we somehow wiggle a charge so that it’s constantly moving but constantly changing that motion—so that it’s accelerating? 5/10/17 Oregon State University PH 106, Lecture #17

Light: Ripples in Space Oregon State University PH 106, Lecture #17 Vibrate a charge and you get a constantly changing electric field—and therefore a constantly changing magnetic field—and therefore a constantly changing electric field... etc. An electromagnetic wave propagates through space (because the fields that are causing one another are properties of space itself). 5/10/17 Oregon State University PH 106, Lecture #17

Oregon State University PH 106, Lecture #17 And the fields are at right angles to each other—and to the direction of the wave’s travel. When we can see it, we call this light. The full name for it (no whether our eyes can see it or not) is Electromagnetic (EM) radiation. Even when our eyes can’t see it, there is lots of it flashing around all the time (more about all that later). 5/10/17 Oregon State University PH 106, Lecture #17

Oregon State University PH 106, Lecture #17 How fast is light—this rippling of fields through empty space? Remember that the speed of a wave depends on the medium—what it’s traveling through and what kind of disturbance the wave was making to the properties of the medium. With sound, the medium was air, and the ripples were disturbances in the pressure and displacement of the air molecules. 5/10/17 Oregon State University PH 106, Lecture #17

Oregon State University PH 106, Lecture #17 With light, the medium is space itself; the ripples are the electric and magnetic fields at each point. Unless we believe that space behaves differently at different locations, it stands to reason that there is one uniform speed with which the field ripples travel through space—and indeed, there appears to be: c = 300,000 km/s 5/10/17 Oregon State University PH 106, Lecture #17

Common Sources of Light Oregon State University PH 106, Lecture #17 Thermal sources (warm objects): If light waves are produced by vi-brating charges, then virtually everything in the universe can give light —because every body has thermal energy: random vibrations of atoms. Other accelerations of charged matter: Gravitational “falling;” nuclear attractions/motions in/around atoms. 5/10/17 Oregon State University PH 106, Lecture #17

Oregon State University PH 106, Lecture #17 Most light is unpolarized—its E-fields and B-fields are vibrating in all directions perpendicular to its motion, because the vibrating (accelerating) charges that cause it are vibrating (accelerating) in all directions without preference. Human eyes cannot detect the polarization of light, but some animals can—which can be useful, since the sun’s light become polarized when it is scattered off of gas particles in the atmosphere. 5/10/17 Oregon State University PH 106, Lecture #17

When Light Strikes Matter Oregon State University PH 106, Lecture #17 The rippling EM fields of light that strike matter can move charged mass around. (After all, those fields exert forces on the charges, which acceler-ate the mass—doing work.) Light can transfer energy to matter. How? It can reflect off it (leaving some of its energy behind). It can transmit through it (leaving some of its energy behind). It can be absorbed completely (transferring all of its energy). 5/10/17 Oregon State University PH 106, Lecture #17