Chapter 14: The Properties of Light


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Historically, understanding the properties of light became the key to understanding the and, hence, to understanding matter. In this chapter we enlarge our concept of light to "electromagnetic radiation" which includes visible light, but also x-rays and other forms that are not visible to the eye.

Electromagnetic radiation has two sources: atoms and charged particles. In 1864, James Clerk Maxwell summarized all that was known about electricity and magnetism into mathematical equations which bear his name. From them he predicted that accelerated charged particles would create electromagnetic (particles, waves?) that would travel at (number?) miles per second through space. Soon radio, and later television, were invented and designed on the concept of electromatnetic waves moving in space.

Galileo had tried to measure the speed of light, but failed. Armand Fizeau measured the speed of light in 1849 and sure enough it was the 186,000 miles per second that Maxwell later calculated. From that time forward through the 19th century, light was understood to be (particles, waves?) . In order of decreasing wavelength, we have
  • waves (up to about a mile in length),
  • television and FM radio,
  • (inches in length),
  • infrared,
  • visible light,
  • ultraviolet,
  • x-rays, and finally,
  • rays (with wavelengths comparable to the diameter of a nucleus.)

    All understood as electromagnetic waves.

    But what is light? Before the 19th century Isaac Newton thought it was a stream of particles. During the 19th century it was thought to be waves. In the 20th century, Albert Einstein pointed to evidence that it was not waves at all, but particles! But how could one not tell the difference? Particles and waves are almost opposite things. Particles are localized lumps of stuff (mass-energy), but waves are spread out disturbances of a medium. How could people not tell the difference?

    Evidence that light is a wave: Light as it passes through a hole. When the size of the hole is about (larger, same size?) as the wavelength of light, the diffraction becomes appreciable. Laser light diffracts noticeably when it passes through tiny slits. Light with itself when laser light from a single source passes through two parallel slits and is seen striking a screen behind in a characteristic patter of light and dark lines (interference pattern).

    Evidence that light is particles: When a photograph is made in reduced light, one can see where the light impacts the film at small, localized points...like impacts of a stream of particles. We call these particles of light .

    The effect is the name describing the experimental observation that light, whether it is either a stream of particles or waves, carries energy and should knock electrons from a piece of metal and thus discharge an electroscope. However, when white, visible light is shined onto a charged electroscope, electrons (are, are not?) discharged. When the white light is made brighter and more intense, the light (does, does not?) dislodge the electrons. This is very puzzling if light is a wave, because the brighter light ought to have a larger wave amplitude and thus carry more energy...enough to dislodge the electrons. But, it doesn't!

    Albert Einstein offered to explain why the experiment failed and how to fix it, but it required giving up the idea that light was waves. Einstein thought of the light as a stream of particles (photons). The energy of each photon was to be

    photon energy = (Planck's constant) x (frequency)


    If light were such a stream of particles, then ultraviolet light (which people who thought of light as waves had assigned a higher frequency and shorter wavelength) should dislodge the electrons. It does, and the photoelectric effect is taken as evidence that light is (particles, waves?) .

    Conclusion: Even though waves and particles are nearly opposite kinds of things, there is evidence that light behaves sometimes waves and sometimes like a stream of particles. That is very, very puzzling. Perhaps it is better to conclude at this point that light is NEITHER a stream of particles nor waves. Light is Light! It's a brand new thing!





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