A paint is a plastic that contains dye molecules
and small clear particles. While the dye molecules absorb certain wavelengths
of light and give the paint its color, the particles are often completely
clear. Nonetheless, these randomly shaped and oriented particles are very
important to the paint’s appearance.
1. The speed of light is different in the particles than it is in the plastic surrounding them. Briefly explain what happens to light as it attempts to pass through these particles in the plastic.
Answer: Light changes speed as it enters a particle and as it exits that particle, so there is a partial reflection from each surface.
Why: Whenever a light wave changes speeds abruptly, part of that
wave reflects. There is an abrupt change in speed as a light wave enters a
particle in the plastic and as that wave subsequently leaves the particle for
the plastic. At each transition, part of the wave reflects.
2. A dyeless paint contains only transparent particles in its layer of plastic. This paint appears white because light emerges from the paint heading in all directions. Why doesn't the light simply go straight through the paint or reflect directly backward?
Answer: The particles in the plastic have surfaces at random angles and partially reflect the light waves passing through them at random angles as well. Light returns from the paint layer traveling in all directions.
Why: Light bounces randomly through the disorderly collection of clear particles. The reflections aren’t directed straight back because the particles’ surfaces aren’t all parallel to the paint layer.
3. Lead carbonate is a clear, toxic chemical that was put in paints before 1930 to make them white. The speed of light in a particle of lead carbonate isn’t that much slower than in plastic. How does this fact help explain why “lead paints” weren’t very good at covering dark surfaces, requiring lots of layers of paint before the surfaces really looked white?
Answer: Since light changes speeds only slightly in entering or exiting from the lead carbonate particles, the partial reflections from the particle surfaces are weak. It takes many, many particles to send most of the light backward so that it doesn’t pass through the paint layer.
Why: To hide the material under the paint, the paint must reflect virtually all of the light before it can pass through the paint layer. But with a weak reflector like lead carbonate, the light must pass through many, many particles before most of it has been reflected, so the paint layer must be very thick.
4. Modern paints contain particles of clear, nontoxic titanium dioxide. Light travels very slowly in this material. Explain why titanium dioxide based paints look so white, even after only a single layer of paint.
Answer: Since light changes speeds dramatically in entering or exiting from titanium dioxide, the partial reflections from the particle surfaces are strong. Only a few reflections are needed to stop most of the light so even a thin layer of paint is needed.
Why: By the time a light wave has passed through a few dozen titanium dioxide particles, it has been nearly completely reflected. Each surface reflection is strong, so not many are needed to stop the wave almost completely.
The electronic flash in a
typical camera is based on a xenon flashlamp, a tube filled with xenon gas at
high pressure with an electrode at each end. This flashlamp is electrically
connected to a small but powerful capacitor to form a circuit so that if the
flashlamp were to conduct current, it would allow current to flow from one
plate of the capacitor to the other.
5. 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 V appears across the xenon flashlamp. The flashlamp, however, conducts no current. Why not?
Answer: The xenon gas contains electrically neutral atoms and they do not accelerate in response to electric fields. There are no charged particles moving the gas to conduct a current.
Why: Even though there is a strong electric field in the flashlamp, there are no charged particles to accelerate in response to that field. The xenon atoms polarize slightly, but not enough to matter. No charged particles flow through the gas so it conducts no current.
6. When you take a picture, the shutter opens and the camera causes a small high-voltage transformer to inject a few electrons into the gas in 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 electrons into the flashlamp cause it to “flash”?
Answer: The electrons accelerate in response to the electric field in the flashlamp. They collide with gas atoms and ionize those atoms, releasing more charged particles. This cascade or chain reaction fills the flashlamp with charged particles so a huge current can flow.
Why: The first few charges introduced into the flashlamp trigger an avalanche of charge particle formation. Collisions knock electrons out of xenon atoms and soon the flashlamp is filled with positively and negatively charged particles—a plasma. Current flows through this plasma and the capacitor’s energy is quickly converted into light.
7. The flashlamp will only last for a certain number of flashes because each flash damages the electrodes. Why does the flash damage the electrodes?
Answer: Sputtering by fast-moving xenon ions gradually wears away the metal in the electrodes.
Why: Each time the flashlamp flashes, ionized xenon atoms go crashing into the negatively charged electrode. They hit so hard that they knock tungsten atoms out of the electrode and deposit them on the walls of the flashlamp. The electrode slowly wears away and the clear flashlamp gradually turns dark.
8. 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: Low pressure xenon emits specific wavelengths of light corresponding to the energies between specific orbitals in the xenon atom. But at high-pressures, the xenon atoms get hit by one another during the process of emitting light and these interruptions allow the atoms to emit a broader variety of wavelengths.
Why: Pressure broadening occurs when collisions upset atoms in the process of emitting light. The collision energy can add or subtract from the energy going into the emitted photon and spread its wavelength out substantially. A much richer variety of wavelengths is emitted by a high-pressure lamp.