. During a recent ice storm, I was standing in my front doorway before dawn and the entire southern sky turned brilliant blue-green for about five seconds or more. What caused this effect? People who missed it tell me it was just a transformer "blowing up" but I've seen one blow up on our street and there is no comparison. The light I saw virtually filled the entire horizon.
You probably saw a sustained high-voltage arc between high-tension wires and/or the ground. I would guess that the ice pulled down one of the wires or caused a tree to fall across them. While transformer explosions often involve hundreds of kilowatts of electric power being turned into light and heat, most of that light is hidden from view inside the transformer. Such an explosion can be dramatic, with some nice sparks and flashes, but it's usually not very bright. However, when a high-tension wire arcs, a significant fraction of the many megawatts of power flowing through the arc is converted directly into light. In effect, a high-pressure arc lamp forms right in the air and it looks like a camera flash that just keeps going until something stops the arc or the power is shut off. The blue-green color you saw comes from characteristics of the air and metal wires involved in the arc. As you saw, a couple of million watts of light are enough to light up the predawn sky quite effectively!
There is, however, an alternative explanation: you may have seen the "green flash" that occasionally appears just as the sun reaches the horizon at sunrise or sunset. This flash is a refraction effect in the atmosphere in which only blue-green light from the sun reaches the viewer's eyes for a second or two while the sun is just below the horizon. However, this green flash should appear in the eastern sky just before dawn, not the southern sky.
. Could you explain the meaning of polarization in optics? Please try to associate it with water waves if possible, to help me visualize it, and avoid the use of electric and magnetic fields. — AM, Yavne, Israel
I can't completely avoid electric and magnetic fields because polarization in optics is associated with a wave's electric field. I also can't depend entirely on water waves because they only have one (transverse) polarization. Still, I will try.
First, consider a wave traveling toward us on the surface of a lake. Suppose that this wave passes under a small boat and I ask you which way the wave is making the boat move. You would tell me that the boat is moving up and down. I would then tell you that the wave is vertically polarized because it causes objects that it encounters to move up and down rhythmically.
Unfortunately, pure water won't do for the next step because it won't support horizontally polarized waves. So let's imagine that some ecological disaster has turned the entire lake into gelatin. An explosion at the side of the lake now causes a wave to begin heading toward us on the gelatin lake, but this strange wave involves a side-to-side motion of the lake's surface. Now when the wave passes under the boat, the boat moves side-to-side rhythmically. In this case the wave is horizontally polarized because it causes objects that it encounters to move left and right rhythmically.
Now let's return to optics. When an electromagnetic wave heads toward us, its electric fields will push any electrically charged particles it encounters back and forth rhythmically. If we watch one of these charged particles as the wave passes it and observe that this particle moves up and down, then the wave is vertically polarized. If instead the charged particle moves left and right, then the wave is horizontally polarized.
. How does the "night vision" mode of the car rear view mirror work? — P
The glass in the rear view mirror is cut so that it forms a thin wedge—it's thicker at the top than it is at the bottom. Its back surface is fully mirrored by a layer of aluminum. For daytime use, the mirror is oriented so that light from behind the car enters the glass, reflects from the layer of aluminum on the back surface, and returns through the glass to your eyes.
But when you tip the mirror upward for night use, the mirrored back surface presents you only with a view of the car's darkened ceiling. However, there is a weak second reflection from the clear front surface of the mirror—whenever light changes speeds, as it does upon entering the glass, some of that light reflects. About 4% of the light striking the front surface of the mirror from behind the car reflects without entering the glass and is directed toward your eyes. Since the image you see is about 25 times dimmer than normal, it doesn't blind you the way a reflection from the mirrored surface would.
. I have heard that there is a substantial cost to starting a fluorescent light fixture. When entering and exiting a room frequently, is it better to leave a fluorescent light turned on, or to turn it off when leaving each time? — GEW
Whenever you turn on a fluorescent lamp, a small amount of metal is sputtered away from the electrodes at each end of the tube. These electrodes are what provide electric power to the gas discharge inside the lamp and sputtering is a process in which fast moving ions (electrically charged atoms) crash into a surface and knock atoms out of that surface. Because sputtering is most severe during start up, a typical fluorescent tube can only start a few thousand times before its electrodes begin to fail. To avoid the expense and hassle of having to replace the tube frequently, you shouldn't cycle the lamp more than once every ten minutes. If you will only be away for a minute or two, leave the lamp on. But if you will be away for more than about ten minutes, turn it off. Incidentally, the claim that a fluorescent lamp uses a fantastic amount of electric power during start-up is nonsense. It's just a myth.
. is a photon a specific unit of measurement of light? Has it been decided if light is a particle or a wave? Why? — J, Australia
There is no doubt about it: light is both a particle and a wave. While it is traveling, light behaves as a wave—for example, it has a wavelength. But when it is being emitted or absorbed, light behaves as a particle—for example, it may transfer momentum, angular momentum, and energy to whatever it hits. A photon is a quantum of light, the smallest packet of light that can exist. You can't have half a photon of light—it's all or nothing. The amount of energy in a particular photon of light depends on the frequency (or wavelength) of that light.
. How would you figure out how much pressure a 100 lb. woman's high heel would produce as she walks? — JB, Boulder, Colorado
If the woman were standing still, with about half her weight on the heel of her right shoe, she would be exerting a force of 50 pounds on the floor under that heel. Since a spiked heel is about 0.33 inches on a side, its surface area is about 0.1 square inches (0.33 inches times 0.33 inches). Since a force of 50 pounds is applied to an area of 0.1 square inches, the pressure on the floor is 50 pounds divided by 0.1 square inches or 500 pounds per square inch. That's about 30 times as much pressure as the atmosphere exerts on objects at sea level.
But when the woman is walking, she often lands hard on that heel, so that it supports her entire weight and then some. The extra force comes about because she is accelerating—when she lands, she is heading downward and the floor must push upward extra hard on her to stop her downward motion. If we suppose that the total downward force she exerts on the heel reaches a peak of 200 pounds—not at all unreasonable—the pressure the shoe exerts on the floor reaches 2000 pounds per square inch. No wonder spiked heels damage floors and present a serious hazard to nearby toes!
. How does an ultrasonic bath work? — PT
An ultrasonic cleaner exposes a bath of liquid to very intense, very high frequency sound. Sound itself consists of regions of high and low pressure that move through a material as waves. As these waves pass through the liquid in the bath, each tiny portion of liquid vibrates back and forth in response to these pressure fluctuations. Near the surface of an object immersed in the bath, the liquid is pushed first toward the object and then away from it. The pressures involved are large and the changes in velocity within the liquid are so intense that occasionally the liquid will actually pull away completely from the object so that a tiny empty cavity forms. In effect, the liquid is jumping up and down on the object's surface and it occasionally jumps so hard that it leaves the surface altogether. Cavities of this sort are unstable and the liquid soon returns to the object. When it does return, the liquid collides violently with the surface and the liquid's pressure skyrockets as it transfers all of its momentum to the object in millionths of a second. This "cavitation" process is what cleans objects immersed in the ultrasonic bath—the dirt and grime are pounded free by the liquid when it returns to fill cavities that have formed during the vibrations.
. How does the tachometer in a new car work? It looks like a magnet wrapped with wire that's located very near a saw-toothed wheel that spins as the engine turns. — TR, Provo, UT
The device you describe is essentially an electric generator. The toothed wheel is made of pure iron so that its teeth can become temporarily magnetized while they are close to the permanent magnet. When a tooth becomes magnetized as it approaches the permanent magnet, or demagnetized as it moves away from the permanent magnet, it changes the shape and strength of the magnetic field around the permanent magnet. Since changing magnetic fields produce electric fields, the tooth's movement causes an electric field to appear around the magnet. This electric field pushes on mobile electric charges in the wire coil wrapped around the magnet and generates electricity. The current in the coil flows one way as a tooth approaches the magnet and reverses when that tooth moves away from the magnet. Also, the faster the tooth moves, the stronger the change in the magnetic field and the higher the voltage generated in the coil. The tachometer can tell how fast the engine is turning by how frequently the current in the coil reverses directions or by how much voltage the coil generates.
. What makes an airplane fly? — SDH, Vicksburg, MS
While there are several ways to understand how air supports a plane's weight, I will look at it first in terms of the deflection of the air flowing past the plane's wings. As the plane moves forward, air flows both over and under the plane's wings. It flows across the wing from its leading edge to its trailing edge. The air that strikes the inclined lower surface of the wing is deflected downward and leaves the wing's trailing edge with a slight downward component to its motion. The air that flows over the arced and inclined upper surface of the wing travels a more complicated route, curving up, over, and down before leaving the wing's trailing edge with a slight downward component to its motion. In both cases, the wing has made the air accelerate downward by pushing the air downward and it is the nature of our universe that the air must push upward on the plane in response. It's a case of action and reaction: if one object pushes on another, the second object must push back on the first object with an equal but oppositely directed force. So the plane's wing pushes down on the air and the air pushes up on the plane. When the plane is moving fast enough and the wings are properly shaped and/or tilted, the upward force that the air exerts on the wings can support the weight of the plane and suspend it in the air.
Another important view of flight involves air pressure in the streams of air flowing over and under the plane. When the air passing under the wing curves downward, it actually does so because the pressure just under the wing is higher than the pressure far from the wing—the air stream is experiencing an overall downward force due to this pressure imbalance and this downward force is deflecting the air stream downward. When the air passing over the wing arcs up, over, and down, it is also doing so because the pressure just above the wing is different from that far from the wing. In this case, the pressure just over the wing's leading edge is quite high—enough to deflect the air stream upward initially. But the pressure over the rest of the wing's upper surface is very low and the air stream curves inward toward the wing; arcing downward so that it leaves the wing's trailing edge with a small downward component to its motion. Overall, there is a low average pressure above the wing and a high average pressure below it. This pressure imbalance produces an overall upward force on the wing and supports the plane's weight.
These two views of flight—one involving deflection of the air stream and the other involving pressure imbalances—are intimately related to one another and really only two descriptions of the same process. Incidentally, the low pressure just over most the wing causes the air flowing over that wing to speed up. That's Bernoulli's equation in action—when air following a streamline experiences a drop in pressure, it accelerates in the forward direction.
. If there was a hole drilled directly through the center of the earth and a ball was dropped into it, what would happen to the ball? Would it oscillate up and down in the hole until it remained suspended in the center? — JC, Dallas, TX
Yes, if the hole were drilled from the north pole to the south pole, the ball would behave just as you say. Assuming that there were no air resistance, the ball would drop through the center of the earth and rise to the surface on the other side. It would then return via the same path and travel all the way back to your hand. This motion would repeat over and over again, with the ball taking 84 minutes to go from your hand to your hand. That time is the same as it would take a satellite to orbit the earth once at sea level. In effect, the ball is orbiting through
the earth rather than around
However, because there would be air resistance unless you maintained a vacuum inside the hole, the ball wouldn't rise to its original height after each passage through the earth. It would gradually loss energy and speed, and would eventually settle down at the very center of the earth.
Finally, the reason for drilling the hole from the north pole to south pole is to avoid complications due to the earth's rotation. If you were to drill the hole anywhere but through the earth's rotational axis, the ball would hit the sides of the hole as it fell and its behavior would be altered.