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The Straight Dope


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Visa cards are printed with little holographic doves as forgery protection, and I've seen similar holographic images printed on things no thicker than a piece of construction paper. Soon there will be chocolate bars with holographic decorations etched on the surface (this according to Scientific American). How are these little holographic pictures made and how do they fool the eye into seeing depth where there really is none? --Susannah Faulhaber, Alameda, California

As is often the case with technical subjects, Susannah, we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or one that is comprehensible but wrong. Being a journalist and therefore shameless, we naturally opt for the latter. What follows is the Ollie North explanation of holography--it might get you past a congressional committee, but don't try it on your PhD board.

A reflection hologram, which is what you find on a credit card, can be thought of as an emulsion containing jillions of mirrored balls, tiny versions of the ones you see at roller rinks. Each facet of each mirrored ball contains a fraction of the whole image. As you gaze at the array of mirrored balls you see a set of facets that contains one perspective of the holographed scene. As you move your eyes to one side, a different set of facets comes into view showing the scene from a slightly different perspective. The changing perspective creates the illusion of three dimensions.

Simple, no? OK, now for a Jack Anderson-like expose of the many lies and omissions contained in the preceding.

L&O #1. There aren't any mirrored balls. Actually each "mirrored ball" is a set of quasi-hyperboloidal interference fringes. Interference fringes reflect a percentage of the light that strikes them. Amounts to the same thing as mirrored balls, but they look a lot different, and from the standpoint of conceptual grabbiness they're strictly to puke.

L&O #2. The change of perspective isn't the only thing that creates the 3-D effect. There's also parallax shift. Your eyes, being two inches apart, look at the scene from slightly different angles, and thus see different sets of "facets." Your brain combines the two images to create one scene with the illusion of depth, just as with a stereoscopic viewer.

L&O #3. I didn't tell you anything about lasers, wave fronts, or coherent light. Do I hear anybody complaining? I didn't think so. However, for those who absolutely must know, I should say that lasers are essential to creating holograms because they're the only known way to create the requisite interference fringes. Memorize the preceding sentence and mutter it under your breath next time some would-be expert (e.g., your precocious eight-year-old) starts to quiz you too closely on the subject. We may not explain everything in this column, but we give you enough to get by.

Once during our vacation last summer my daughter demanded a swim in the pool immediately after dinner. I told her to wait at least 30 minutes. Being of the age (12) that no longer accepts what I say as gospel, she insisted on an explanation. "Because my parents made me do it" was the best I could do on short notice. Was I right to insist she wait? Or was I conned by my parents? --Joe Nadeau, Oshkosh, Wisconsin

If she wants to splash in the pool, let her splash. From the 1930s through the 1950s there was a great fear that swimming after eating would lead to stomach cramps that would double you over in agony, causing you to sink like a stone. This was thought to be a leading cause of drowning. As late as 1956 the Red Cross water-safety manual devoted several pages to the topic complete with staged photo of a gasping "victim." That same year, however, U. of Georgia swim coach B.W. Gabrielsen published a book called Facts on Drowning Accidents that revealed that swimming after eating was implicated in fewer than 1 percent of drownings. Thereafter the wait-an-hour hysteria began to subside. It's now thought stomach cramps are rare. It still isn't a good idea to do strenuous swimming right after eating lest you exhaust yourself. But a quick dip in the pool after dinner is harmless.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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