The Feynman Lectures on Physics, Vol. I: The New Millennium Edition: Mainly Mechanics, Radiation, and Heat (Volume 1)

by Richard P. Feynman

Resolving power of a grating

The parabolic antenna

Colored films; crystals

Diffraction by opaque screens

The field of a plane of oscillating charges

31 The Origin of the Refractive Index

The index of refraction

The field due to the material

The correct picture of an atom, which is given by the theory of wave mechanics, says that, so far as problems involving light are concerned, the electrons behave as though they were held by springs.

Dispersion

The phenomenon that the index depends upon the frequency is called the phenomenon of dispersion, because it is the basis of the fact that light is “dispersed” by a prism into a spectrum.

Absorption

The energy carried by an electric wave

Diffraction of light by a screen

Diffraction by a screen.

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Radiation Damping. Light Scattering

Radiation resistance

But from the standpoint of circuit theory, without considering where the energy goes, the net effect on the circuit is the same—energy is “lost” from that circuit. Therefore the antenna appears to the generator as having a resistance, even though it may be made with perfectly good copper.

This resistance that an antenna shows is called the radiation resistance.

The rate of radiation of energy

Radiation damping

Independent sources

Scattering of light

We have just explained that every atom scatters light, and of course the water vapor will scatter light, too. The mystery is why, when the water is condensed into clouds, does it scatter such a tremendously greater amount of light?

So lumps of atoms radiate or scatter more energy than they do as single atoms. Our argument that the phases are independent is based on the assumption that there is a real and large difference in phase between any two atoms, which is true only if they are several wavelengths apart and randomly spaced, or moving. But if they are right next to each other, they necessarily scatter in phase, and they have a coherent interference which produces an increase in the scattering.

How many atoms can we put together before we cannot drive this argument any further? Answer: If the water drop gets so big that from one end to the other is a wavelength or so, then the atoms are no longer all in phase because they are too far apart. So as we keep increasing the size of the droplets we get more and more scattering, until such a time that a drop gets about the size of a wavelength, and then the scattering does not increase anywhere nearly as rapidly as the drop gets bigger. Furthermore, the blue disappears, because for long wavelengths the drops can be bigger, before this limit is reached, than they can be for short wavelengths.

33 Polarization

The electric vector of light

The real and imaginary components of a complex electric field vector are only a mathematical convenience and have no physical significance.

Polarization of scattered light

Birefringence

Polarizers

Optical activity

The intensity of reflected light

Anomalous refraction

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Relativistic Effects in Radiation

Moving sources

Finding the “apparent” motion

Synchrotron radiation

The electromagnetic radiation emitted by relativistic charged particles circulating in a magnetic field is called synchrotron radiation.

Cosmic synchrotron radiation

Bremsstrahlung

The Doppler effect

The four-vector

Aberration

The momentum of light

Therefore, when light is shining on a charge and it is oscillating in response to that light, there is a driving force in the direction of the light beam. This is called radiation pressure or light pressure.

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