BASIC GEOMETRICAL OPTICS
Scenario—Using Geometrical Optics in the Workplace
Manuel Martinez is a photonics technician hired recently to work for a large opticalcompany that manufactures optical components such as mirrors, lenses, prisms,beam splitters, fibers, and Brewster windows—all to customer specifications. Whilein school Manuel studied light imaging with mirrors and lenses, ray tracing, andcalculations with simple formulas. After two months on the job he has discoveredthat he uses those same ideas day in and day out. To be sure, things are muchmore “high tech” in his company, for now Manuel has access to powerfulcomputers and computer programs that trace rays through complicated opticalsystems, often containing elements with nonspherical surfaces, something Manuelnever had a chance to do at school. He enjoys the challenge of using state-of-theartlab equipment he’s never seen before, including autocollimators,spectroreflectometers, and surface profilers. All in all, he’s really satisfied becauseall of the optics he had in his “Geo” course back at school really prepared him wellfor his laboratory work here. This month Manuel is learning how to “grind and polishoptical surfaces to spec,” and how to apply the principles of geometrical optics todetermine when the surfaces are “near tolerance.” Manuel finds his workfascinating and can hardly wait to get to work each morning. “Geo” was never somuch fun.
Note: The hands-on exercises that follow are to be used as short introductory laboratory demonstrations. They are intended to provide you with a glimpse of some of the phenomena covered in this module and to stimulate your interest in the study of optics and photonics.
1. Comparing Ordinary Light with Laser Light. In an appropriately darkened room, and with plenty of “chalked-up” erasers, examine the dramatic difference between ordinary “flashlight” light and laser light. Use a focusable mini MAGLITE (MAG Instrument, Ontario, Canada, 909-947-1006 begin_of_the_skype_highlighting 909-947-1006 end_of_the_skype_highlighting) and a well-collimated, ordinary low power (5.0 mW or less) diode laser pointer (Edmund Scientific Company, Barrington, New Jersey, 609-573-6250 begin_of_the_skype_highlighting 609-573-6250 end_of_the_skype_highlighting). Shine each light beam, in turn, from one side of the room to the other. Have participants “pat the erasers” together over the entire path of the light beams. The light beams outline themselves dramatically as they scatter their light energy off the settling chalk particles. Which beam remains well defined along its path? Which beam more closely describes a “ray of light”?
2. Bending Light Rays in a Fish Tank. Fill an ordinary rectangular five-gallon acrylic fish tank half full of water. Use the diode laser pointer to trace a “light ray” through the water in the fish tank.
Attach the laser—generally cylindrical in shape—to a stand, making sure that it can be directed easily in different directions. From above the tank, direct a beam onto the top of the water at an angle of incidence near 50°. (A plane mirror placed under the tank will reflect more light back into the water.) See sketch D-1 below. Use milk or a food coloring (very sparingly–a drop at a time) to illuminate the beam. Experimenting beforehand—with a smaller container—to determine the right amount of coloring will pay big dividends. With the ray visible in the tank, observe the bending of the light beam as it moves from air into water, the phenomenon of refraction.
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