From: "Frank Underdown Jr." Subject: Re: Ray optics, Geometrical optics and Wave optics Date: Fri, 08 Oct 1999 17:58:03 GMT Newsgroups: sci.physics.electromag,sci.optics,sci.physics,sci.math.num-analysis Dany wrote in message <37fdfaca.25833310@news.et.tudelft.nl>... > >Dear All Tutors, > >We have followed colleges, the three terms >Ray Optics (RO), >Geometrical Optics (GO) and >Wave Optics (WO) >are often appeared in the our textbooks. >However, none of these boos explained what are >the different between them. These confuse us a lot, ______________________________________________________ Greetings: Geometrical and ray optics are the same. In ray optics you assume that the wavelength of the light goes to zero and treat the controlled manipulation of rays by refraction and reflection at an interface neglecting ad diffraction effects (Hecht 2nd ed. p. 129) Wave optics or physical optics deals with the situation where the wavelength of light is nonzero. Since light is an electromagnetic wave the completed description of its propagation through a medium is contained in Maxwell's equations which takes into consideration the wave nature of light by use of the wave equation. cheers, Dr. Frank Underdown Jr. Physics Dept. Michigan Tech. University funderdown@portup.com ============================================================================== From: skeckhardt@mmm.com (Steve Eckhardt) Subject: Re: Ray optics, Geometrical optics and Wave optics Date: 8 Oct 1999 18:13:32 GMT Newsgroups: sci.physics.electromag,sci.optics,sci.physics,sci.math.num-analysis In article <37fdfaca.25833310@news.et.tudelft.nl>, derijken@mailcity.com says... >Dear All Tutors, >We have followed colleges, the three terms >Ray Optics (RO), >Geometrical Optics (GO) and >Wave Optics (WO) >are often appeared in the our textbooks. >However, none of these boos explained what are >the different between them. These confuse us a lot, >Can some Tutors out there please take a litle time >to explain and help us to distinguish them. I had the same experience as you when I was beginning to learn optics, so I may be able to help. First, let me add to your confusion. The most rigorous theory of optics is quantum electrodynamics. This theory is exact to the limits of our ability to measure. Blackbody radiation, line spectra and the photoelectric effect are examples of things that require quantum theory. Richard Feynman has an excellent and approachable book on the subject called Q.E.D. Unfortunately, this theory is so complicated when all of the mathematical detail is included that it can be used only when the number of quanta involved is relatively small. For most problems, we must therefore resort to approximations. The next most general theory of optics is based on Maxwell's equations. It is sometimes referred to as vector diffracton theory. All experiments that do not involve the quantization of light energy can be explained by this theory. Exact calculations of the diffraction of light by an aperture or diffraction grating, and coupling of light into an optical fiber require vector diffraction theory. Propagation of a rapidly converging or diverging beam of light is another example in which vector diffraction is important. Again, the mathematics is so complicated that only a few simple problems can be exactly solved. Continuing to rougher approximations, we arrive at scalar diffraction, which is also known as wave optics. The approximation is that the fine details of the polarization of the light can be ignored; light is assumed to be polarized perpendicular to the direction of travel of the beam or unpolarized. With this approximation we can still model the propagation of a laser beam and interference effects to good precision. Diffraction can be modeled well enough for most engineering efforts. Propagation of light in a waveguide is approximated with acceptable accuracy. Using this theory, most simple optical systems can be modeled with a reasonable amount of math. Only when complicated optical systems must be modeled to we need to resort to the next approximation. If we assume that the wavelength of light is much smaller than the smallest detail of the optical system we wish to model, we arrive at geometrical optics. This is synonymous with ray optics. Even the most complicated optical systems can be modeled in this approximation. Lens design and illumination are the major application of this approximation. This is the bread and butter for the hundreds of optical engineers in the U.S. There is one more level of approximation in optics: it is called first order, paraxial or Gaussian optics. In this approximation, all of the angles between the rays of light and the optical surfaces are assumed to be small enough that the trigonometry can be dispensed with by assuming sin(x) = tan(x) = x. First order optics is what is used to sketch out optical systems. It is the basis for the "Lensmaker's equation" that is taught in high school and college physics classes. Considering how far it is from the exact nature of light, the most amazing thing is how closely it predicts things such as image location and size. -- Best Regards, Steve Eckhardt (skeckhardt@mmm.com) Opinions expressed herein are my own and may not represent those of my employer.