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?
(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?
(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?
(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?
(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?
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?
(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?
(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?
(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?
(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?
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?
(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"?
(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?
(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?
(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?
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?
(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?
(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).
(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?
(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?
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?
(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?
(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?
(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?
(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?