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Forces of Nature

WebLecture: Light and Matter

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Forces of Nature Weblecture

Light and Matter


Cohesion and Chemical Affinity — Faraday Lecture IV

Faraday continues his exploration of the forces of chemical affinity by demonstrating a variety of reactions between materials, some producing heat, some light, and some new materials.

Electromagnetic Radiation: the speed of light and the wave nature of light

Speed of light

The inability to time experiments accurately hampered Galileo's efforts to determine the speed of light because he was working before the invention of a reliable clock, and the distances over which he could send a controlled beam were relatively short. Roemer's determination of the speed of light from observations of the moons of Jupiter were possible only because the distances involved were large enough to make the differences between predicted events and observations of the event noticeable. Fizeau-Foucault's method has been the most reliable method, and modern measurements still rely on experiments that use coherent beams of light (LASERS, usually), and rapidly rotating mirrors.

Because of its accuracy, Michelson and Morley used this system of light beams and rotation mirrors to attempt measuring the rate of speed of the earth through the "ether" that they believed filled all of space by measuring the difference in the speed of light when sending light in the direction of the earth's motion and at right angles to it. This resulted in the most famous experimental failure of all time, because they detected no difference! Ultimately, they had to conclude that not only was there no ether, but that the speed of light was constant regardless of the motion of its source, a point of data which Einstein explained in his theory of relativity. We'll come back to this result when we discuss the implications of relativity for stars of enormous mass that collapse to black holes.

The wave nature of light

Electromagnetic radiation is simply a form of propagation of energy through space, usually from a point source. Isaac Newton conceived of light as small particles that moved through space; different colors had different properties. However, Christaan Huygens thought of it as a form of wave. Experimental evidence to support Huygen's theory was supplied by Thomas Young's double slit experiments, which showed that light going through a pair of narrow slits produces a diffraction pattern similar to that of waves in water passing through similar barriers.

James Clerk Maxwell put observations by Auguste Coulomb, Andre Ampere, and Michael Faraday together to create a coherent picture of electromagnetic radiation. Coulomb's rules explained that two electrical charges worked like two masses. Where two masses experience an attractive force proportional to the masses and inversely proportional to the square of the distance between them, two charged particles have a force (whether it is attractive or repulsive depends on the type of charge) proportional to the charges and inversely proportional to the distances. Ampere noted that current in wires resulted in magnetic forces. Faraday was able to show that changing magnetic fields cause electric current to flow. Maxwell realized that only when charges are static (not moving) are they without magnetic effect, but that a changing electric field will generate a changing magnetic field, that can in turn generate a changing electric field. Such a phenomenon would be a self-propagating wave (see diagram on page 104). When Maxwell calculated the speed of the wave, he realized that it coincided with the speed of light, and concluded that the self-propagating wave was light.

For another presentation of this material, you may like to look at the University of Winnepeg's physics site on the Wave Nature of Light. There is a good explanation of the physics of the double-slit experiment and diffraction from the University of Tennesee Physics Course.

The frequency and wavelength of any wave (light, sound, water waves) are related by the formula c = λ * ν, where c is the speed of the wave, λ is the wavelength or distance between successive peaks, and ν (sometimes f) is the frequency, or number of peaks passing a given point per second. The longer the wavelength, the fewer the number of peaks passing a given point, so the lower the frequency. We say that frequency and wavelength are inversely related: λ = c/ν and ν = c/λ.

Atomic Structure

Review my chemistry weblecture on theories of matter at the theories of matter at the Scholars Online Chemistry website. Faraday is part of the nineteenth century effort to understand and apply Dalton's laws of chemical affinity, and to discover and identify the elements, a contribution that eventually led to Mendelyeev's table of elements.

Take a look at the Physics Classroom website on Waves, Sound and Light.

  • On Geometrical Optics: The Physics Classroom: Ray Optics.
  • On Atomic Structure: Atomic Archive: Introduction to Atomic Physics (9 pages).