. I know that an electromagnetic wave cannot pass through the holes in a metal cage (a Faraday cage) if those holes are significantly smaller than the wavelength of the wave. But what if it is just a constant electric field? What determines the hole size now? — KBH, Logan, Utah
If the electric field isn't changing with time, then it can't enter a metal cage no matter how large the cage's holes are. In effect, the constant electric field has an infinite wavelength and can't propagate through holes of any finite size. However, the holes don't stop an electromagnetic wave instantly—the wave does penetrate a short distance into the cage before it dwindles to insignificance. The distance over which the wave diminishes by a factor of about 3 is roughly the size of the hole through which it is trying to pass. So if your Faraday cage has holes that are 1 centimeter in diameter, the constant electric field will take several centimeters to diminish to nearly zero. If the holes are much larger than that, the electric field will penetrate far into the cage and the cage will only be an effective shield if it is extremely large. To avoid having to use a very large cage, it's better to use small holes.
. What is infrared light? — AC, Teaneck, NJ
Infrared, visible, and ultraviolet light are all electromagnetic waves. However these waves differ in both their wavelengths (the distances between adjacent maximums in their electric fields) and in their frequencies (the number of electric field maximums that pass by a specific point in space each second). Infrared light has longer wavelengths and lower frequencies than visible light, while ultraviolet light has shorter wavelengths and higher frequencies than visible light. We can't see infrared or ultraviolet lights because the cells of retinas aren't sensitive to these lights. Nonetheless, we can often tell when those lights are present—we may feel infrared light as heat on our skins and we may find ourselves sunburned by ultraviolet light.
. Why does light travel slower in some media than in a vacuum? For example, in glass or other transparent media, visible light is not absorbed and yet it slows down. What's going on? — FH, Waltham, MA
When a light wave enters matter, the light wave's electric field causes charged particles in the matter to accelerate back and forth. That's because an electric field exerts forces on charged particles. The light wave gives up some of its energy to these charged particles and is partially absorbed in the process. However, the charged particles don't retain the light's energy very long. They are accelerating and accelerating charged particles emit electromagnetic waves. In fact, they reemit the very same light wave that they absorbed moments earlier. Overall, the light wave is partially absorbed and then reemitted by each electrically charged particle it encounters, so that the light continues on its way as though nothing had happened.
However, something has happened—the light wave has been delayed ever so slightly. This absorption and reemission process holds the light wave back so that it travels at less than its full speed. If the charged particles in the matter are few and far between, this slowing effect is almost insignificant. But in dense materials such as glass or diamond, the light wave can be slowed substantially.
Actually, higher frequency violet light is slowed more than lower frequency red light because violet light is more effectively absorbed and reemitted by the atoms in most transparent materials. That's because when a high frequency light wave encounters the electrons in an atom, the jiggling motion is so rapid and the electrons' motions are so small that the electrons never reach the boundaries of the atom. As a result, those electrons are able to jiggle back and forth as though they were free electrons and they do a good job of slowing the light wave down. But when a low frequency light wave encounters the electrons in an atom, the jiggling motion is slower and the electrons' motions are so large that they quickly reach the boundaries of the atom. As a result, those electrons aren't able to jiggle back and forth as far as they should and they don't slow the light wave down as well.
. What is ink made of? — JD, Langley, British Columbia
Ink is made of light absorbing pigment particles or dye molecules that are suspended in a fluid that contains a dissolved binder chemical. When the ink is deposited on a sheet of paper, the binder's solvent diffuses into the paper or evaporates into the air, leaving the pigment particles or dye molecules bound to the paper by the binder.
. Can you suggest an experiment to prove that a helium balloon floats because helium is lighter than oxygen? — CR
If you have a balance scale, you can do a series of comparisons. First compare a cup of water to a cup of salad oil, using the balance, to show that the salad oil is less dense than the water. Then show that the salad oil floats on water. Then compare an air-filled balloon to an identical helium balloon, using the balance, to show that the helium is less dense than air. Then show that the helium floats on air. It's just like the salad oil on water, but now it's the helium on air. You can't simply pour the helium on the air to show that it floats, because they'll mix. So you leave the helium wrapped up in a rubber balloon and then let it float on air. It floats just fine!
. I've heard that, technically speaking, our atmosphere is a fluid. Can you discuss this?
Since both gases and liquids are fluids, the earth's atmosphere is certainly a fluid. Any material that flows in response to sheer stress (tearing) is considered a fluid. The earth's atmosphere flows in responses to sheer stress—for example when you drive your car past another car, the air in between experiences this tearing and it flows in a complicated fashion. Winds are another important example of fluid flow in the earth's atmosphere.
. When raisins are added to a solution containing water, baking soda, and vinegar, why do the raisins dance? — RE, Troy, IL
Baking soda and vinegar react in water to release carbon dioxide molecules. If the chemicals are sufficiently dilute in the water, the carbon dioxide molecules may remain dissolved in the water almost indefinitely. But when the water has impurities in it, the carbon dioxide molecules tend to come out of solution as gas bubbles at those impurities. The impurities allow the molecules to form tiny gas bubbles—a process called nucleation. In the present case, the raisins serve as the impurities that nucleate gas bubbles. As the gas bubbles grow on the surfaces of the raisins, the raisins experience upward buoyant forces from the surrounding water. The bubbles float upward, carrying the raisins with them and causing the raisins "to dance."
. When an object is free falling, I understand that the earth's gravity causes its velocity to increase at 10 meters/second2
in the downward direction. Is there a point at which this object would reach a "terminal velocity" in the earth's atmosphere and cease to accelerate? — CS, Sykesville, MD
Yes, most objects will reach a terminal velocity and stop accelerating downward. The faster an object drops, the more air resistance it experiences. This air resistance pushes the object upward and at least partially cancels the downward force of gravity—the object's weight. When the object's downward speed becomes high enough, the upward air resistance force exactly cancels the object's downward weight. At that point, the object experiences zero net force and it no longer accelerates. Instead, it descends at a constant downward velocity—its terminal velocity. This terminal velocity is determined partly by the object's density and size and partly by its aerodynamics. Large, dense, and aerodynamic objects tend to have very large terminal velocities while small, low-density, non-aerodynamic objects tend to have very small terminal velocities.
. If you have four carts of equal weights, one with small wheels, one with large wheels, one with small wheels in front and large wheels in back, and one with large wheels in front and small wheels in back, which cart will be easiest to move? — PK
The cart with the small wheels will be easiest to move. That's because, as the cart starts moving, each kilogram of mass in the wheels acquires twice as much energy as each kilogram of mass in the cart itself. Keeping the mass of the wheels low by making the wheels small reduces the energy in the overall cart and makes it easier to start or stop.
. How does the temperature of a fire correspond to its color. How hot is blue fire? How hot is yellow fire? — SF, Lake Almanor, CA
The hotter the fire, the more green and blue light it emits. The dimmest glow that you can see in a darkened room appears when a surface is about 400° C. The dull red of a heat lamp is about 500° C. A candle's yellow glow is about 1700° C. A normal incandescent lamp is about 2500° C. And the sun is about 5800° C. Blue fire would be hotter still, except it's usually colored artificially by the presence of excited atoms. Atomic emissions are colored because atoms can't emit all colors in order to produce a normal spectrum of thermal radiation. Instead, they preferentially emit only specific colors. That's why when you burn copper, you see blue-green light, even when the copper isn't very hot. The copper atoms just can't emit red or yellow light, even though those would be the more appropriate colors at the temperature of the burning copper.