. Please explain the "Wagon Wheel Effect." How can the wheel appear to move forward, then backward, then stop, just by viewing it differently? — J, Davenport, IA
This effect is the result of viewing a series of stop-action frames in rapid sequence as a movie or video. Even though a wagon wheel is turning forward, its orientation during sequential frames of a movie may make it appear to be stopped or turning backward. For example, if the wagon wheel completes exactly one full turn between each frame of the movie, the wheel will appear to be stopped—its orientation in each frame will be the same. If it completes slightly less than one full turn between each frame, it will appear to be turning backward! As you can see, a tiny change in wheel rotation rate, from slightly more than one full turn per frame to slightly less than one full turn per frame, is enough to make the wheel appear to switch from turning forward, to stopped, to turning backward. So it's no wonder that the wheels appear to change speeds abruptly from no apparent reason.
. Is it harmful for children to sit too close to microwave ovens? Is it possible to get "burned" opening the microwave oven during a cycle or too soon after a cycle? I realize the oven shuts off, but is there residual radiation? — C
As long as the microwave oven hasn't been damaged and doesn't leak excessive microwaves, there should be no harm in having children sit near it. I wouldn't hold my face right up against the door edges because that would be asking for trouble with leakage, but it's extremely unlikely that even doing that once in a while would cause injury.
As for being injured by microwave radiation after the cycle has stopped, that's essentially impossible. As soon as the high voltage disappears from the magnetron tube and it stops emitting microwaves, the microwaves in the cooking chamber begin to diminish. Even if they bounce 1000 times off the metal walls of the chamber before they're absorbed by those walls or the food in the microwave, that will only take about 2 millionths of a second. You can't open the door fast enough to let them out before they're already gone.
. Could you please give me a precise explanation of light scattering in relation to blue moons and red sunsets. Do dust particles, or whatever, facilitate the transmission of some wavelengths and not others? — DW
While the expression "blue moon" usually refers to the infrequent occurrence of second full moon in a calendar month, there have been rare occasions when the moon truly appeared blue. In those cases, an unusual fire or volcanic eruption filled the air with tiny clear particles that had just the right sizes to resonantly scatter away the red portion of the visible light spectrum so that only bluish light from the moon was able to pass directly to the viewer's eyes. The moon thus appeared blue.
Red sunsets are much more common and they are caused by Rayleigh scattering—the non-resonant scattering of light by particles that are much smaller than the light's wavelength. While Rayleigh scattering is rather weak, it's weaker for long wavelength light (red light) than it is for short wavelength light (violet light). As a result, blue and violet lights are scattered more than red light; making the sky appear blue and the sun and moon appear red, particularly when they are low on the horizon and most of their blue light is scattered away before it reaches your eyes. When there is extra dust in the air, such as after a volcanic eruption, Rayleigh scattering is enhanced and the red sunsets are particularly intense.
. How does a foghorn turn on and off? — M, Brant Rock, MA
Although I am not certain, I would guess that most automatic foghorns detect the fog optically. They either send light from a source to a detector and turn on the foghorn when the detector fails to see the light or they send light into their surroundings and turn the foghorn on when they see excessive reflection of that light.
. How do you make lasers that burn?
Lasers use excited atoms or atom-like systems to amplify light. Putting mirrors around such excited atoms or atom-like systems allows them to amplify their own light until the laser is emitted vast numbers of identical light particles or "photons." To burn something with laser light, there must be a great many excited atoms or atom-like systems and they must be very efficient at amplifying light. Probably the easiest to build powerful laser is a carbon dioxide laser. This laser uses an electric discharge in a mixture of nitrogen and carbon dioxide gas to produce excited carbon dioxide molecules. These molecules amplify infrared light at a wavelength of 10.2 microns extremely efficiently, so that a laser consuming about 1000 watts of electric power can emit approximately 100 watts of infrared light. That's enough power to burn things very quickly. Even more powerful carbon dioxide lasers are used in industry to cut and machine metals, including thick steel plates. But while they are surprisingly simple to build and operate, given the right components, carbon dioxide lasers require dangerous high voltage power supplies. There were many physics graduate students electrocuted in the 1960's while tinkering with homemade carbon dioxide lasers.
. How does a sound dish work? I know that it's a parabola, but I can only find drawings not explanations. — DW, Omaha, NE
A sound dish is actually a mirror telescope for sound. When sound waves from a distant source encounter a rigid parabolic surface, they reflect in such a way that they focus to a point. If you put a microphone at that point, it will detect the sound waves from the distant source. You can see this focusing effect by drawing a parabola on a sheet of paper and directing parallel lines—the sound waves from the distant source—toward the parabola. If you reflect each line in a mirror-like fashion from the surface it hits, you'll find that all the reflected lines pass through a single point as they move away from the parabola.
. Do crystals "grow"? — JJ
Yes, crystals grow. They begin as tiny seed crystals when a few atoms or molecules manage to arrange themselves accidentally in an orderly fashion. From that point on, new atoms or molecules that join the seed continue the orderly pattern and the crystal grows. Some crystals grow from solutions that contain more of particular atoms or molecules than those solutions can handle. Other crystals grow from molten materials that are cooling off and beginning to freeze. Still other crystals form from atoms or molecules that are diffusing randomly through solids, liquids, or even gases and encounter the proper crystal on which to stick.
. How are the nylon ropes of parachutes able to stop the falling parachuter? How much of a force must they over come, and how might the ropes' elasticity be affected? — C
When the parachuter opens the parachute and begins to slow down, the parachute's nylon shrouds briefly exert a large upward force on the parachuter. Over a period of a few seconds, the parachuter slows from a downward speed of about 150 mph to a downward speed of 20 mph and experiences several g's of upward acceleration. To cause this much upward acceleration, the nylon shrouds must exert an upward force on the parachuter that is several times the parachuter's weight. The nylon shrouds are quite strong and can easily tolerate this much tension without exceeding their elastic limits. There should be no adverse effects on their elasticities.
. I'm doing a science experiment of what factors affect the distance a golf ball travels. One of my factors is the bounciness of the ball. Does this have any effect on the distance the ball will go? — EG, North Salem, NY
Yes. The bouncier the golf ball, the farther it will go after being struck by a golf club. While we normally think of a bounce as occurring when a ball hits a stationary object, it's also a bounce when a moving object hits a stationary ball. The golf ball bounces from the golf club and the more bouncy the golf ball is, the faster and farther it will travel.
. How does gravity influence the passage of time? — AW, Karachi, Pakistan
Gravity's effects on time are the result of general relativity. Any concentration of mass/energy curves the space/time around it, which is ultimately why objects passing near that mass/energy are deflected. This curvature of space/time also slows the passage of time for objects that are near the concentration of mass/energy. To see why this slowing of time must occur, imagine people operating a radio transmitter on the surface of a very massive planet. They transmit their radio wave at exactly 100 MHz. You are far from the planet with your radio receiver and you begin trying to find their transmission. You will find it at a lower frequency, perhaps 99 MHz. That's because their radio wave has had to struggle to escape from the planet's gravity and has used up some of its energy in the process. Since energy is proportional to frequency, the radio wave shifts toward lower frequency as it climbs out of the planet's gravitational well to reach your receiver. Since the people on the planet think that their system is operating at 100 million oscillations per second and you think that it is operating at only 99 million oscillations per second, the people on the planet are evidently experiencing time more slowly than you are. Their second actually lasts longer than yours.
To understand how their time passes more slowly that yours, you can think of the radio wave's frequency as the ticking of a clock. The time it takes the clock's ticks to reach your ear isn't important in measuring the passage of time. What you care about is how often those ticks occur. When you "listen" to the ticking of the clock on the big planet, it ticks 99 million times each second. However, to the people on the planet, it ticks 100 million times each second. This apparent inconsistency is explained by the fact that time is passing faster for you than for the people on the planet. Their second lasts longer than yours, which is why they count more ticks during their second than you count during your second.