Using the Schlieren Optics setup (refer to the description given) with most of the room lights turned off, place an object in front of the mirror. (3) An alternative experiment dramatically demonstrates the diffraction of light around edges but does not show the structure of the interference patterns. However, the real thing definitely looks better than the video image so, if it's a relatively small class, invite the students to come up and see for themselves after lecture. The details in the shadow are quite sharp albeit dim and close-up video camera/projection is necessary for audience visibility. The object is positioned a couple of meters from the screen. The arc lamp is placed far from the object (and screen) to more closely approximate a point source - the arc is a couple of millimeters long. Nevertheless, it is not temporally coherent light. It is, of course, quasi white light in the sense that it's a discreet multi-line source rather than a continuous spectrum and it's color balance is heavy toward the blue end. (2) Interference fringes can also be demonstrated with a point white light source. Ideally one would start with the object (razor blade) far from the laser (and thus totally illuminated) and then slide it close to the laser (on the optics rail) to blow up the pattern - "animated" in this manner, the audience will have a better sense of what is being shown. It's a compromise between blowing up the detailed interference pattern and still recognizing the object producing the effect. 1 The razor blade is positioned so that at least an entire corner and edge are illuminated the closer to the laser, the larger the magnification on the screen, but less of the blade will be imaged. Although a microscope objective can be used as a beam expander, by far the most satisfactory results are obtained using a spatial filter. (1) It is necessary to diverge and expand the laser beam so that it illuminates an appreciable area of the object of choice - traditionally this has become a razor blade. However, dramatic effects can be produced with white light and these too will be discussed (2 & 3 below). The most readily available light source meeting these prerequisites is, of course, the laser and the demonstration using a laser will be described first (it also works the best). Although monochromatic light (temporally coherent ) produces sharper detail, white light works very well too. Narrow interference bands are seen within the shadow of a straight edge while more complicated shapes yield more complicated interference bands and striations.Ī point light source (spatially coherent light) is a necessity to see the effect. A point light source will produce seemingly sharp shadows which turn out to be not at all sharp when viewed under magnification.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |