I.  Light Waves and Physical Optics

In our study of ray optics and image formation, we represented image points as "geometrical points," without physical extent. That, of course, followed logically since light rays were used to locate the image points and light rays are lines that intersect clearly at geometrical points. But in reality, if you were to examine such image points with a microscope, you would see structure in the "point," a structure explained only when you invoke the true wave nature of light.

In effect, then, we are saying that, with large objects such as prisms, mirrors, and lenses—large in the sense that their dimensions are millions of times that of the wavelength of light—interference and diffraction effects are still present in the imaging process, but they occur on so small a scale as to be hardly observable to the naked eye. To a good approximation, then, with "large" objects we are able to describe light imaging quite satisfactorily with geometrical (ray) optics and obtain fairly accurate results. But when light waves pass around small objects, such as a 100-m-diameter human hair, or through small openings, such as a 50-m pinhole, ray optics cannot account for the light patterns produced on a screen beyond these objects. Only wave optics leads to the correct interpretation of such patterns.

And so now we turn to a study of the wave nature of light and to the fascinating phenomena of interference, diffraction, and polarization—and of such devices as gratings and thin-film coatings. We shall see that interference occurs when two or more light waves pass through the same region and add to or subtract from each other. Diffraction occurs when light waves pass through small openings or around small obstacles and spread, and polarization occurs due to the transverse nature of the electric field vibration in a propagating electromagnetic wave. Before we look at these phenomena, let's review briefly the nature of waves, wave fronts, and wave motion.