|MadSci Network: Physics|
Greetings: The process of interference between electromagnetic waves ranging from radio waves to light waves is used for a large number of applications. Holography is one of the most interesting uses of interference. Microwave holograms are being used to study antennas and to search for hidden weapons on airline travelers and light wave holograms are used to prevent tampering with passports, drivers licenses and now there is a new brand of candy with editable reflection holograms molded into each piece. Interference occurs when waves of the same frequency cross each other. They can be moving in the same direction, in opposite directions or at an angle to each other. When the position of the crests and troughs of two waves match each other they add and we obtain a bright spot and when the position of a wave crest matches a wave trough the waves cancel each other and we get a dark spot. A hologram is composed of a two or three dimensional array of millions of these interference points. I have plotted the development of simple one dimensional interference patterns between two waves traveling in opposite directions in the answer to a question to Mad Science: Physics : Re: When waves collide, do both kinds of interference occur? Adrian Popa, Staff Optical/Microwave Physics, Tue Jan 21 11:42:59 1997 The basic optical hologram is formed from the interference between two light beams obtained from a common source, usually a laser. One beam directly from the laser is called the reference beam and the second beam is reflected from or transmitted through an object and is called the object beam. The interference between these two beams forms the hologram which is usually captured on photographic film. The difficulty in making a hologram is to keep the distance between the object, the film and some of the optics from moving less than about 1/10 of a light wavelength during the several seconds (about 1 to 30 sec.) required to expose the film. This stability usually requires expensive optical tables, lens and mirror mounts and higher power lasers. A large amount of information on holography with dozens of links can be found at the HOLOGRAPHY LINKS PAGES: http://www.enter.net/~holostudio/links.html There are three techniques that I would recommend you use to make holograms of simple objects that are not as clear as those made on expensive equipment; however, these techniques do demonstrate the process and they form three dimensional images. The first technique was developed by one of our Mad Scientists and you can form simple images from holograms scratched into Plexiglas. This technique has minimal cost for it does not require a laser or film. The experiments are found at the following Web page: http://www.eskimo.com/~billb/amateur/holo1.html The second technique requires a laser and film to make WHITE LIGHT HOLOGRAMS. Small, complex objects can be viewed in 3-D using a white light bulb or a projection lamp to illuminate the film. High resolution film and a laser are required to make the hologram. The reference for this technique is: "The Amateur Scientist: How to stop worrying about vibration and make holograms view able in white light", Scientific American Magazine, May 1989, pages 134-137. Scientific American Magazine is available in most of the larger libraries. If you cannot obtain a copy of the article send me an e-mail note and I can send you a copy. I would recommend that you use the glass mounted 649 type film and use a simple red diode laser pointer that you can obtain at Radio Shack stores for about $30 or you can buy various pieces of equipment, lasers, optics etc. for making holograms and also a hologram setup from the Edmund Scientific Company, 101 E. Gloucesster Pike, Barrington, NJ 08007-1380 (telephone: 609-547-8880 or e-mail
. They will send you a free catalog with lots of neat science stuff for thousands of experiments. Obtaining the film and developing it will be your most difficult task. I would not do fast drying of the film with a hair dryer as suggested in the article, letting the hologram dry slowly for a day will provide the most crisp images, patience pays off. Also: you will never see a hologram while the film is wet, you must have patience and wait for the processed film to dry from 12 to 24 hours. Also; I have found that making images of clear glass ship models or glass animals placed very close to the front or back of the film is the best way to start your experiments until you get good holograms. The glass objects reflect lots of light reducing the exposure time. Third Technique I have made very nice holograms using a laser and only one lens to expand the laser beam to illuminate both the film and the object (as shown below) eliminating the need for 2 beams. This type of ZERO ORDER HOLOGRAM considers the part of the beam missing the object and lighting the film to be the reference beam. The light reflected from and transmited through the object to the film is the object beam. By placing the small glass object in front of the film, almost touching it, vibration effects are greatly reduced and also gives short exposure times, providing the highest probability for success. Don't worry about shadows of the object on the film until you make your first sucessful hologram. In later experiments you can move the object away from the film to remove the shadow or go on to more complex setups.. 10X OR 20X MICROSCOPE OBJECTIVE LENS FILM PLATE /\ / / --> -------------------I I / I / I I\ / ___I___ / PARALLEL LIGHT I I \ / \GLASS / / RAYS COMING FROM I I \ / \SHIP/ / LASER I I \ / / ----------------------I----I-------FP-------------/--- AXIS I I / \ / I I / \ / I I / \ / I I/ \ / --> -------------------I I \ / \/ SHORT / <--FOCAL--> / ON PLAY BACK LENGTH VIEW HOLOGRAM FROM HERE SIMPLE HOLOGRAM SETUP (DISTANCE NOT TO SCALE) Best regards - your Mad Scientist - Adrian Popa
Try the links in the MadSci Library for more information on Physics.