. When light hits an object, how do we recognize the color? — CM, Levering, PA
White light is a mixture of various light waves with different wavelengths and thus different colors. When white light hits an object, some of the light waves are absorbed while others are not. The light that isn't absorbed may pass through the object or it may be reflected in a new direction. The light that you observe coming from the object is this transmitted or reflected light. If the light that you see doesn't include the same mixture of wavelengths that first hit the object, you won't see this light as white. Instead, you'll see it as colored. If the light you see contains mostly long wavelengths of light, you'll see it as red. If the light contains mostly short wavelengths of light, you'll see it as blue or violet. The wide range of colors that objects have comes from subtle differences in the wavelengths of light they absorb. However, when an object is illuminated with colored light, the light that it transmits or reflects may be altered. After all, it can't transmit or reflect a light wave that never hit it in the first place. Even variations in "white" light can affect an object's color—makeup looks different in incandescent "white" light than it does in fluorescent "white" light because those illuminations contain different mixtures of light waves.
Lasers use systems with excess energy to amplify light. These systems, typically atoms or atom-like structures in solids, are in excited states—they have more than their minimum amounts of energy. An excited system can get rid of its excess energy in many different ways, but certain systems tend to emit the excess energy as photons—particles of light. While an excited system will emit a photon spontaneously if you wait long enough, it can also duplicate a passing photon if that passing photon has the proper characteristics. Most importantly, the excited system must be naturally capable of emitting the passing photon spontaneously—the passing photon's wavelength and travel path must be such that the excited system is able to duplicate it.
This duplication effect makes it possible to amplify light. When a single photon passes by a number of identical excited systems, those systems may duplicate the photon many times so that many identical photons emerge. This phenomenon is the basis for laser amplifiers. When one of the photons emitted spontaneously by the excited systems is deliberately sent back and forth through those systems with the help of mirrors, the laser amplifier becomes a laser oscillator—it both initiates and amplifies the light. The light that ultimately emerges from the laser oscillator or amplifier differs from normal light because the laser light consists of many identical photons. They all have identical wavelengths (colors) and follow identical paths through space. They also exhibit dramatic wave effects, particularly interference.
. What is the chemical formula for glass? — GL, Birmingham, AL
Glass isn't a simple molecule that can be represented by a normal chemical formula. It's a network solid in which the atoms are joined in one gigantic non-crystalline structure. In effect, a piece of glass is a single enormous molecule. Window glass is called soda-lime-silica glass and consists mostly of silicon, oxygen, sodium, and calcium atoms. Silicon and oxygen are considered to be network-forming atoms and bind to one another in long atomic linkages that form the backbone of the glass. The sodium and calcium atoms are added to terminate the linkages. This network termination softens the glass, lowers its softening and melting temperatures, and generally makes the glass easier to work with. Harder glasses such as lead "crystal" replace the sodium and calcium with other materials (e.g. lead oxide) that don't weaken the glass as much and produce harder or stronger glasses. Pyrex cookware contains boron instead of sodium and calcium, and is a borosilicate glass.
. How can MRI pictures show slices through an object? And how do you get an image from using a magnet?
MRI images show where hydrogen nuclei (protons) are located in a person's body. Protons are magnetic particles that have only two possible states in a magnetic field: aligned with the field or aligned against the field (also called "anti-aligned"). This limited range of alignments is the result of quantum physics. Normally, the protons in a person's body are equally divided between aligned one way and aligned in the opposite way. But when a person is placed in a strong magnetic field, the protons in their body tend to align with the magnetic field and the distribution of aligned and anti-aligned protons shifts. There are then somewhat more aligned protons than anti-aligned protons.
Once there are more aligned protons than anti-aligned protons, it becomes possible to flip them about. Flipping these protons from aligned to anti-aligned takes energy and this energy can be provided by a radio wave. But not just any radio wave will do: its frequency must be just right in order to provide the proper amount of energy or the proton won't flip. When the right radio wave is provided, some of the aligned protons will flip to become anti-aligned. This flipping of protons can be detected by a sensitive radio receiver.
By placing the person in a non-uniform magnetic field and by adjusting the frequencies and timings of the radio waves, an MRI device can determine where protons are located in the person's body to with a few millimeters. A computer records where the protons are and then displays information about them as cross sectional images. For example, the computer can display a dense concentration of protons as white and a region with few protons as dark. MRI is particularly good at imaging tissue because tissue contains lots of hydrogen atoms and their protons.
. Does the air pressure of a basketball and the hardness of the floor surface have an effect on the height of the bounce? — BB, West Unity, OH
Yes to both questions. When a basketball collides with the floor, the ball's kinetic energy—its energy of motion—is temporarily stored as elastic potential energy in two objects: the ball and the floor. The fractions of the collision energy stored in the basketball and the floor depend on how far each of them dents—the more one dents, the larger the fraction of the collision energy it receives. How well the basketball rebounds from the floor depends on how much of the collision energy returns to the ball during the rebound. Some of the stored energy in each dented surfaces is converted to thermal energy and is lost from the bouncing process. A hardwood floor is very springy and returns its share of the collision energy efficiently. A properly inflated basketball is also very springy. Thus when a firm basketball bounces on a good hardwood floor, it bounces well. But if the basketball is underinflated, its surface bends too far so that it receives most of the collision energy and internal friction in the ball's skin wastes most of that energy. The ball bounces weakly. And if you try to bounce the ball on a soft carpet, the carpet dents easily, receives most of the collision energy, and wastes most of it as thermal energy. Again, a weak bounce.
. How do helicopters work? — KH, Holland, MI
The wings of a normal airplane obtain upward lift forces from the air as the airplane moves forward through the air. That's because the shape and angle of the wings is such that air flows faster over the top surface of each wing than under the bottom surface of that wing and the air pressure above the wing drops below the air pressure below the wing. Each wing experiences a net upward pressure force and these upward forces are enough to support the weight of the plane.
A helicopter spins its wings around in a circle so that they move through the air even when the helicopter itself is stationary. Normally, these rotating wings are called blades. Again, the air flows faster over each blade than beneath it and there is a net upward pressure force on each blade. These upward forces support the helicopter and they also allow it to tilt itself—by adjusting the angle of each blade as the blades turn, the helicopter can obtain twists from the air so that it tilts one way or the other. Once the helicopter has tilted, it can use some of the lift force from its blades to push it horizontally so that it accelerates forward, backward, or toward the side.
. How do I graph (line or pie) the time it takes different amounts of water to freeze? — LC, TX
First, you must determine what it is that you're really measuring. If you pour a gallon of water onto a huge copper plate that's been cooled to -200° C, the water will freeze in a fraction of a second while if you put a drop of water on a hot frying pan, it will never freeze at all. You must design a sensible experiment and then repeat it with several different amounts of water. The experiment should be sure to focus on the water by avoiding situations where external effects determine the freezing time. For example, you might obtain 4 identical 1-liter containers and fill them with 1/4, 2/4, 3/4, and 1 liter of the same water respectively and then put them simultaneously in a freezer with a uniform cold temperature. Then you can record how long it takes each of them to freeze. Then use an XY graph to plot these times: the x-axis could be the amount of water in the container and the y-axis could be the time it took for the water to freeze. The four points you'll obtain probably won't form a straight line. That's because the amount of heat that must leave the water for it to freeze depends on the water's volume and the time it takes that heat to leave depends on the water's surface area. Doubling the water's volume doesn't double its surface area, so the freezing time will have an interesting and somewhat complicated dependence on the water's volume. Try it!
. I have heard of a magnetic top that will spin on top of another magnetic field because of the gyroscopic effect. If that is put into a vacuum chamber, would it spin perpetually? — JH, Visalia, CA
Probably not. The magnetic top that you mention is a wonderful invention, sold under the name "Levitron". It uses gyroscopic precession to stabilize what is normally an unstable arrangement: two oppositely aligned magnets, one supporting the other. In air, you can get the Levitron top to stay aloft for a couple of minutes before its spin rate declines to the point where it stops being stable. In a vacuum, I'd expect it to last much longer but not forever. Thermodynamics overwhelms just about everything sooner or later and the Levitron won't be an exception. Even if you get rid of air resistance, the spinning top's strong magnetic field will interact with its environment and will allow the top to exchange energy with that environment. While there is always the possibility that these exchanges will make the top spin faster, such favorable exchanges are relatively unlikely. Instead, the energy exchanges are much more likely to extract energy from the top and slow it down. For example, any conducting surfaces near the Levitron top will exert a magnetic drag force on the top and will convert its energy into thermal energy in those conducting surfaces. Forever is a long time and the top will certainly slow to a stop eventually. Still, it might be interesting to see how long it can stay spinning. I'll bet 10 minutes is the realistic maximum. If I have a chance to test it out, I'll let you know what happens.
. My son and I are building an electromagnet for a science project. We know that if we wrap the wire around the nail and connect the battery to the wire...presto, a magnet is born. But what is it about flowing current that allows this to happen? — GG, Westfield, NJ
Moving electric charges are inherently magnetic. That's because electricity and magnetism are intimately related and aren't really separate phenomena. To see why this is true, imagine two electrons sitting motionless in front of you—they push one another away with electric forces. But now imagine that you and those two electrons are moving northward in a train and someone standing beside the track is watching all of you pass. From that person's perspective, the two electrons are moving and they exert both electric and magnetic forces on one another. What appears to you to be a purely electric effect appears to the person near the track to involve both electricity and magnetism. Without the appearance of magnetic effects in moving charges, grave inconsistencies would appear in the dynamics of objects view from different perspectives.
So the current in the wire of your electromagnet is inherently magnetic. The magnetic field it produces aligns the tiny magnetic domains in the steel nail so that the nail's magnetic field greatly strengthens that of the current in the wire.
. If microwave cookers are so energy-efficient, why can't similar machines be used as hot water heaters or in central heating systems? - GB
Microwave ovens transfer about 50% of the electric energy they receive from the electric company to the food. Conventional ovens transfer only something like 10%. Cooking just isn't a very energy efficient process because you're trying to get heat into an object from outside that object. In contrast, an electric space heater transfers 100% of the electric power it receives to the room around it. Home heating is much more energy efficient because you're getting heat into an object from inside that object. In effect, your microwave oven is also 100% efficient at heating your room—every bit of electric energy it consumes eventually enters your room as heat. But it's an expensive sort of "space heater" and you do better just to use conventional heating systems.