Due Monday, April 3, 1995, In Class
Please Answer Each Question As Briefly As Possible
You May Work Together, But Write Up Your Answers Separately
Question 1:
Paints are usually plastics that contain tiny pigment particles. These particles may absorb certain wavelengths of light, creating colored paints, or they may be perfectly transparent. Paints containing only colorless pigment particles appear white. The classic "white" pigment was lead carbonate but this deadly compound is no longer used, replaced completely by nontoxic titanium dioxide. Let's take a look at what happens when light encounters a layer of paint containing only colorless pigment particles.
(A) If the paint's surface is very smooth, making it a glossy paint, a few percent of the light striking that surface is reflected as though from a mirror. Why?
Answer: Light changes speed as it moves from air to plastic and some of it reflects from the boundary.
Why: Whenever light changes speeds, it experiences refraction and reflection. In this case, the reflection is most important. A small fraction of the light reflects from the surface. The larger the change in speed, the more reflection occurs. For transitions from air to plastic, the reflection is about 4%.
(B) Most of the light enters the plastic surface and then begins to pass through the randomly shaped and oriented pigment particles. These pigment particles are perfectly clear and don't absorb any light, but they have an extremely high index of refraction. What happens to the light as it passes through one of these pigment particles?
Answer: The light refracts and reflects from each surface of the particles.
Why: Again, changes in light's speed cause reflections and refractions. The light does not have an easy time in passing through the clear particles, even though those particles do not absorb it. The light's path changes at each surface and a good bit of it is reflected back out of the paint.
(C) The random shape and orientation of the pigment particles is what gives the paint its "white" appearance. If all of the pigment particles were instead flat disks that lay parallel to the paint's surface (like coins lying flat on the bottom of a shallow pan of water), how would this change the way light reflects from the paint?
Answer: The paint would reflect as from a mirror rather than from a white surface.
Why: The flat disks would reflect light directly back from the paint, in the manner of a mirror. The paint would like very much like a metal mirror rather than a mixture of plastic and clear pigment particles. The random surfaces of the pigment particles are what give white paint is diffuse glow. A narrow beam of light hitting white paint scatters in all directions while a beam of light hitting the strange disk-filled paint would bounce back as a narrow beam.
(D) If the clear pigment particles had exactly the same index of refraction as the plastic that holds them, how would this alter the appearance of the paint?
Answer: The paint would appear completely clear.
Why: Without the changes in refractive index, light passes through the clear mixture unaffected. Although some light will reflect from the air to plastic transition, whatever light actually enters the paint will continue on all the way to the material under the paint. The paint will appear clear. Some semi-gloss and dull finished paints use such "invisible" pigment particles. Although these particles are not visible directly, they make the paint's surface rough and spoil the shiny look of plastic.
(E) Colored paints contain pigment particles that absorb certain wavelengths of light but have indexes of refraction that are similar to those of the plastic. Why must these paints also contain the colorless, high index of refraction pigments found in white paint?
Answer: Otherwise the light that is not absorbed will travel all the way to the material below the paint.
Why: A paint that only absorbs certain colors of light, and that does not scatter the remaining light back out of the paint, will appear transparent. Transparent stains do exactly that. But most paints are not suppose to be transparent; you are not supposed to see the wood or metal underneath the paint. So these paints include the high index of refraction particles to ensure that the light never makes it to the supporting material. Paints with large amounts of white pigment are very nearly opaque, even when only one coating is applied. Paints are rated according to their hiding ability.
Question 2:
Microwave ovens allow for some interesting cooking and funny disasters.
(A) Baked Alaska is a dessert where a hot, baked meringue contains cold, frozen ice cream. The reverse is Frozen Florida, a dessert in which cold, frozen meringue contains boiling hot liqueur. Frozen Florida is prepared by taking a frozen meringue ball (cooked egg whites) containing liquid liqueur (a water-alcohol mixture that remains liquid at low temperature) out of the freezer and putting it in a microwave oven briefly. Why does the liqueur get hot while the meringue remains frozen?
Answer: The liqueur contains liquid water which absorbs microwaves and becomes hot. The meringue contains frozen, immobile water molecules which cannot absorb microwaves.
Why: The water molecules in ice cannot turn back and forth to absorb microwaves and convert their energy into internal energy. Ice and frozen meringue do not become hot in a microwave oven.
(B) If you break an egg into a glass dish, leaving the yolk's tough outer membrane intact, and put it in the microwave, you risk having the yolk explode. What causes this explosion?
Answer: The yolk will boil and the increasing pressure inside the membrane can cause it to explode.
Why: The microwaves heat the water molecules inside the yolk without overheating and damaging the tough membrane around the yolk. While conventional cooking usually weakens the membrane long before it makes the yolk boil, that is not so with microwave cooking. If the yolk overheats and releases large numbers of gaseous water molecules, those water molecules can create an enormous pressure inside the membrane. When the membrane finally bursts, the steam throws gooey yolk all over the inside of the oven. Not a pretty sight.
(C) If you put a sealed aluminum can filled with soda in the microwave oven and turn the oven on for a few minutes, what will happen to the soda and why?
Answer: Virtually nothing happens to the soda can because its aluminum skin reflects microwaves.
Why: Aluminum is a very good conductor of electricity. In the presence of microwaves, the electric charges inside aluminum begin to move and they reflect the microwaves almost perfectly. A tiny amount of microwave power is absorbed because of currents in the aluminum losing energy to heat. Since no microwaves ever get to the soda inside the can, the soda remains cool.
(D) While most glazed ceramic dishes are "microwave safe," some fancy china has metallic decorations. These gold or silver decorations serve as poor conductors of electricity. What will happen to a metal-decorated dish if you put it in the microwave and why?
Answer: The decorations can become very hot because of currents driven through them by the microwaves.
Why: Metallic decorations have free charges that move in the presence of microwave fields. The microwave's electric field pushes those charges back and forth in the decorations. However, the charges suffer many collisions and lose energy to heat. The decorations become hotter and hotter. Eventually, the hot decorations damage the dishes.
(E) The door of a typical microwave has a clear glass window that is covered by a metal screen. Why can't microwaves get through that screen to cook food on the outside of the window?
Answer: Microwaves cannot pass through holes that are significantly smaller than their wavelengths.
Why: All electromagnetic waves are prevented from passing through holes in conducting materials if those holes are much smaller than their wavelengths. Thus an FM radio wave (wavelength about 3 m), cannot pass well through a small window in a metal-walled room. On the other hand, light (wavelength about 0.0000005 m) can pass through a small window. Microwaves (wavelength about 12 cm), can pass through a small window but not the metal screening on it. The holes in the screening are just too small and currents flowing in the metal screen reflects the microwaves.
Question 3:
Your new camera contains an electronic flash. This flash emits light from its xenon flashlamp; a glass tube filled with high pressure xenon gas, one of the noble gases found in small quantities in the earth's atmosphere. This flashlamp has an electrode at each end and is electrically connected to a small but powerful capacitor. These two components form a circuit so that any charge moving from one plate of the capacitor to the other must pass through the flashlamp.
(A) Before you take a picture, the camera places separated electric charge on the two plates of the capacitor until a voltage drop of about 300 volts appears across the xenon flashlamp. But the flashlamp does not conduct any current. Why not?
Answer: Gases of neutral atoms do not respond to electric fields.
Why: Gases are usually very good electrical insulators. They have no mobile electrons so they do not respond to electric fields. To get them to conduct electricity, you must introduce charged particles into the gases.
(B) When you take a picture, the shutter opens and the camera causes a small high-voltage transformer to inject a few ions into the gas inside the flashlamp. The lamp suddenly allows current to flow from one plate of the capacitor to the other and the lamp "flashes". Why does this introduction of ions into the flashlamp cause it to "flash"?
Answer: These ions accelerate in the electric field and collide with gas atoms and the electrodes. These collisions create more and more charged particles and a discharge occurs.
Why: The fields used in a flashlamp are so strong that it only takes a few ions to cause a chain reaction to occur. Each charged particle crashes into other particles and releases more charged particles. Very soon there are so many charged particles around that they gas becomes a good conductor of electricity.
(C) The flashlamp will only last for a certain number of flashes because each flash damages its electrodes. How does the flash damage the electrodes?
Answer: It sputters atoms from the electrodes.
Why: Since discharges involve moving ions as well as electrons, they involve damaging collisions with the electrodes. These collisions cause sputtering, where atoms are knocked right out of the electrode's surface by an incoming ion. Sputtering is particular severe as a discharge starts and ages the electrodes quickly.
(D) The flashlamp uses high pressure xenon rather than low pressure xenon. Why does high pressure xenon give a more uniform spectrum of light than low pressure xenon?
Answer: At high pressure, radiation trapping and/or pressure broadening make the spectrum more uniform.
Why: At low pressure, xenon atoms will emit light only as they move from one of the important states to another important state. But at high pressure, many other states become active so that other wavelengths of light come out of the gas. Collisions occurring during light emission distort the states and cause light at various different wavelengths to be emitted. Also, radiation trapping can prevent some of xenon's favorite wavelengths of light from escaping from the gas so that the more obscure wavelengths of light become more important. Overall, the light becomes more uniform.
(E) The flashlamp uses xenon gas rather than sodium gas, in part because xenon emits light over a very broad range of wavelengths and does a good job of simulating sunlight. But why wouldn't a high-pressure sodium vapor flashlamp be very practical, even if you didn't care that it was orange in color?
Answer: Sodium is a solid at room temperature.
Why: You would have to heat the sodium lamp to high temperature prior to each use. At room temperature, it will not have any sodium in the vapor so it will not work properly. That is why sodium vapor lamps must always warm up to begin working well.
Question 4:
As a DJ at a very small local radio station, you often find yourself involved in technical issues for the station's two channels, one of which is AM at 1020 kHz and the other of which is FM at 89.5 MHz. The station is in the process of building a new transmitting system.
(A) To save money, the director wants to use a single antenna for both channels. You warn him that the AM channel needs a taller antenna than the FM channel. Why is that true?
Answer: The AM channel emits waves with a longer wavelength than the FM channel. Since the antenna's should be on the order of the wavelength of the radiation it emits (usually about 1/2 a wavelength), the AM Channel needs a longer antenna.
Why: Antennas work best when they are about 1/2 as long as a wavelength of the radiation they emit. That gives the charge just about enough time to get to the end of the antenna before turning around to head back. Since the wavelength of the AM transmission is longer than that of the FM transmission, it needs a longer antenna.
(B) The director had planned to put the antennas next to the station, which is in a valley at the base of a small mountain. You suggest putting them at the top of the mountain, despite the extra cost of wires. Why is altitude important, particularly for the FM antenna?
Answer: Radio waves travel principally in a straight lines and do not go through objects well. An elevated antenna has a better line-of-sight to most receivers than one in a valley.
Why: Reception is always best when you can see the transmitting antenna. That is easiest when it is on a hill or mountain.
(C) A suggestion to orient the antennas horizontal is quickly dismissed as non-traditional. But it wouldn't work well, either. List two reasons why a horizontal antenna will produce a radio signal that most people will find hard to receive with their radios? (Hint: one has to do with the orientation of their antennas and the other has to do with their locations relative to the transmitter's antenna).
Answer: A horizontal antenna emits horizontally polarized radio waves that you can only pick up well on a horizontally oriented antenna. The horizontal transmitter antenna does not emit radio waves well along its axis.
Why: The charge on a horizontal transmitting antenna moves horizontally, creating horizontally polarized radio waves. These waves, with a horizontal electric field, can only push charge back and forth on a horizontal receiving antenna. From the ends of the transmitting antenna, the charge of that antenna appears to move toward you or away from you. There is no way you can hold your antenna to receive a strong signal. Although you can receive the signal if you are very close to the transmitting antenna, there are actually no radio waves emitted directly along it so the reception quickly disappears as you move farther away.
(D) The AM channel must be careful not to "overmodulate" the radio wave during very loud passages because it distorts the sound people hear in their radios. You explain this effect as due to moments when the transmitter actually turns itself completely off. Why would the transmitter stop transmitting any wave at all?
Answer: To tell the receiver to move its speaker cone farther away from the listener, the AM transmitter reduces its transmission intensity. If the station tries to move the speaker cone too far from the listener, it will stop transmitting altogether.
Why: The AM station tells the receiver where to place its speaker cone by adjusting the intensity of its transmission. If the station tries to move the speaker cone too far, it will have to transmit very large and very small intensities. It may easily exceed its maximum transmission power capacity on the high side or hit zero transmission power on the low side. Oops.
(E) The FM channel must also avoid overmodulation during loud passages because it will get in trouble with the FCC. Other FM stations in your area will also be angry with your station for spoiling the reception of their transmissions. How can your FM station affect those other FM stations when they are on different channels?
Answer: To tell the receiver to move its speaker cone toward or away from the listener, the FM transmitter changes the frequency of its transmission. If the station tries to move the speaker cone too far, it will emit frequencies that are too far from its official frequency and will begin to emit frequencies owned by other stations.
Why: An FM station is permitted to use only a certain range of radio frequencies. If it exceeds that range, it is in trouble. Since sound is represented with changes in frequency, it must try not to exceed is allowed range of frequencies.
Question 5:
The computer that you won in the charity raffle has a brilliant 17" color monitor with all the bells and whistles. The monitor works just like a fancy television, except that it displays computer information rather than a video signal. You set the monitor on your desk, turn it on, and begin to play with its controls.
(A) When you turn the brightness knob, the image becomes lighter or darker. What is happening inside the monitor's picture tube?
Answer: You are changing the numbers of electrons striking the inside surface of the picture tube each second.
Why: When you turn up the brightness, you are increasing the number of electrons in the electron beam. These electrons are what deliver energy to the phosphors on the inside of the picture tube screen so that the phosphors emit light. When you turn down the brightness, you are reducing the number of electrons in the beam.
(B) There are knobs that control the image's horizontal and vertical sizes. As you increase the image's horizontal width, how is the monitor changing what it does with the two horizontal deflecting coils?
Answer: The electrical currents passing through the coils are increased.
Why: The current in the two horizontal deflecting coils varies rapidly from a certain amount in one direction to a certain amount in the other direction. As the current sweeps from one value to the other, the magnetic field of those coils varies and the electron beam is swept across the screen horizontally. But when you increase the image's horizontal width, the sweeping distance must increase and so must the strength of the magnetic field. The current now varies between two amounts, both larger than before. As a result, the beam starts its sweep farther to the left on the screen and finishes its sweep farther to the right.
(C) The monitor has a "degauss" button. When you press that button, the image suddenly begins to move about and the colors go bananas. These fluctuations gradually diminish in strength and eventually stop altogether, leaving the image clear and sharp, with no color problems. The monitor has demagnetized its shadow mask. Why will a permanent magnetization in the shadow mask cause color problems for the monitor?
Answer: If the shadow mask has a permanent magnetization, its magnetic field will deflect the electrons passing through it and they will strike the phosphor screen at the wrong positions.
Why: Magnetic fields exert forces on moving electric charges. The electrons passing through the shadow mask expect to be in a field-free region so that they can travel at constant velocity until they hit the screen. But a magnetized shadow mask will exert forces on those electrons so that they accelerate and miss their intended targets.
(D) The monitor's instructions remind you not to leave a fixed image displayed steadily for a very long time because that image will "burn into the screen." What physical changes in the screen will result from long exposure to the same image?
Answer: The steady exposure to the electron beam will gradually damage the phosphor coating on the inside of the picture tube.
Why: The electrons that deliver energy to the phosphors can actually damage the phosphors. While most of their energy is used to create light and heat, some energy occasionally ends up ripping an atom or molecule out of the phosphor. This tiny fragment travels across the empty picture tube and deposits itself elsewhere. The remain phosphor coating is not quite as good as it was. Over time, the phosphor coating degrades. If the images move constantly, as they do in television, this degradation is barely noticeable. But if it occurs mostly in one location, as it can with a computer image, the screen can acquire a visible pattern of damage.
(E) Your monitor has a very high refresh rate, meaning that it recreates the image on the screen as many as 100 times each second. What is it doing faster in order to recreate the image so quickly?
Answer: It is scanning the electron beam down the screen (moving back and forth horizontally as it does) 100 times each second.
Why: To keep you from seeing the scanning process, the monitor must recreate its image many times each second. You can see a flicker if it recreates the image only 60 times a second. So the monitor sweeps across and down very quickly, completely rebuilding the image 100 times a second.
