Q: Why can't my eyes focus properly underwater? Jamie Smith, Denver, Colorado
A: When light moves from one medium to another, as it does when it travels from an object, through the air, into your fluid eyeball, the change in resistance makes it bend. A good thing, considering the human eye has evolved to accommodate precisely these circumstances: The fluid behind the cornea refracts light onto the lens; the lens refracts it again and focuses it clearly onto the retina. Underwater, however, is another story. Say you're in a river, washing downstream toward a boulder. Light reflected off the rock travels through the water, but now, because there's no change in resistance, it doesn't bend when it hits the eye. Instead, the lens refracts the light as it does above waterÑonly not enough to correct the off-target ray and snap it into focus. The result is temporary farsightedness: The boulder appears as a fuzzy blob. "Our eyes," says National Eye Institute ophthalmologist Richard Hertle, "were only meant to be bathed in tears."
Q: After the winter solstice, why do days get longer on one end (the evening) rather than on both ends symmetrically? Sam Permut, Scarborough, New York
A: Glad you asked, Sam. It's a simple case of astronomical attraction and declination. Ahem. The earth moves in an elliptical orbit, and in the weeks around the winter solstice (December 21) it draws closest to the sun. The sun's gravitational pull creates a slingshot effect, accelerating the earth through a hairpin turn. At this point, the earth is moving fastest past the sun, spinning on its axis in the same direction it orbits (counterclockwise), meaning your home must rotate about 361 degrees to face back at the sun. That extra degree makes solar noon (when the sun is directly overhead) occur later each day, and sunrise and sunset shift later as wellvisualize sunrise taking one step forward into day, and sunset taking one step forward into night. But because soon after the winter solstice northern hemisphere days begin to get longer in both directions (sunrise taking one step toward afternoon, sunset taking one step toward night), the net result is that sunset takes two steps forward, and sunrise takes one step forward and one step back. In short, sunset gets later but sunrise holds steady. Between January 1 and 10, for example, the sun rises at the same time each morning but sets one minute later each evening. Declination, or the earth's angular distance above or below the sun's equator, plays a role as wellÑbut youÕre off the hook on this one. "It isn't a huge effect," says Pat Palmer, professor of astronomy at the University of Chicago, "and December in Chicago is bad enough anyway."
Q: Bloodsucking insects transport deadly parasites, right? So why don't they die in the process? Michael Brigham, Essex, Vermont
A: "If a parasite kills its host," explains Marc Klowden, a Univer-sity of Idaho entomologist, "it's usually a dead end for the parasite." To avoid suicide, a parasite (bacterium, protozoan, virus, or worm) can either lurk in an invulnerable zone in the insect, stay in the host during a harmless phase of life, or exit the insect fast, allowing safe transmission of the freeloader. Consider the single-cell malaria parasite and its chief carrier, the common Anopheles mosquito. Once ingested by the insect, the parasite mates and grows nonthreateningly in a protective cyst for ten to 14 days. When it bursts from the cyst, instead of fatally infecting the mosquito, it kindly allows itself to be flushed out through the hypopharynx, the hollow skin-piercing lance on the skeeter's head, with the next bite. One disease in which the host is toast, however, is the plague. When a flea feeds on, say, an infected dead rat, it ingests Yersinia pestis. This bacterium blocks food from entering the flea's stomach. The flea then swallows so much indigestible food that, alas, there's no place for it to go but back up. The flea vomits on its victim and spreads Yersinia pestis's deadly pathogens, maybe bites a few animals or people to further spread the disease, and eventually gets its cruel reward: death by starvation.
Q: What causes an ice-cream headache?Beth Dayton, Salem, Oregon
A: The dull, throbbing agony that starts at the top of the throat, peaks after 30 to 60 seconds, and puts a vise grip on the brain for up to five minutes is the result of too much coldany almost-frozen substancehitting a spidery network of nerves that stretches across the roof of the mouth. Swill glacial runoff or inhale your frozen margarita and this network, the spheno-palatine ganglion, spasms. In shock, the nerves misattribute the source of pain to something in your head, telling blood vessels in your brain to dilate in that all-too-painful way. Blame such faulty wiring on the lack of frosty foods in nature: Despite your preference for an apr's-surf Sno-Kone, your palate is wired for the wild. Overcome the pain, says Robert Smith, founder of the Cincinnati Headache Center at the University of Cincinnati College of Medicine, by sipping a chaser of warm water, which relaxes the ganglion. Or avoid it by warming the food on your tongue for a moment, heating your Nalgene in the sun, orheresy!eschewing ice cream altogether.
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