February 23, 2004
Chapter 12, Case 2, a-e
p. 410, Bloomfield
2. 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.
Answer: a) In the xenon flashlamp, the gas tube is connected across the two plates of the capacitor as shown in the figure below. Prior to taking the picture, electric charge is placed onto the plates of the capacitor to create a ~300 V potential across the capacitor. When the voltage is placed across the capacitor, why doesn’t any current flow through the flashlamp? In order to answer this, we first need to consider what it means to conduct current. As we know, current is the flow of charge from one place to another. In this problem, we have a large accumulation of charge at the ends of the capacitor, so why doesn’t any of that charge flow through the flashlamp? To determine whether or not any current will flow through a particular medium, we first need to consider what that medium is made of. In this case, we have a gas of electrically neutral xenon atoms. Since the xenon atoms are neutral, we know that the gas contains no free charge carriers. This means that there is no way to transfer charge between the two ends of the capacitor, that is to say, the xenon gas acts as an insulator between the two plates of the capacitor. Therefore, the flashlamp conducts no current because the atoms in the xenon gas are electrically neutral and so there are no free charges to move between the electrodes.
Figure a) Xenon Flashlamp. When the flashlamp is turned on, charge accumulates on the capacitor plates to create a ~300 V potential drop across the gas tube. Since the xenon atoms are electrically neutral, there are no free charges to carry current through the gas tube (the xenon gas acts as an insulator between the two plates).
Answer: b) Prior to injecting the electrons into the flashlamp, current will not flow through the gas because the gas contains no free charge carriers. When the electrons are injected into the gas, the gas can conduct electricity because it will contain free electrons that can move through the gas. Now that the gas can conduct electricity, the circuit between the two plates of the capacitor is complete, and the 300 V potential drop between the plates will drive the electrons to flow through the gas. But how do these electrons flowing through the gas cause the flashlamp to flash? First of all, we know that the flowing electrons will get a large kinetic energy boost from 300 V potential. These fast moving electrons will travel through the gas between the two electrodes, but when the electrons move through the gas, they are jostled about as they collide with the xenon atoms (in the same way that electrons traveling through a wire will get bounced around by the atoms in the wire). When the electrons collide with the xenon atoms, they transfer some of their kinetic energy to the atoms. These collisions between the electrons and the xenon atoms must somehow be responsible for causing the flash. So what happens during the collision that creates the flash? Every once in a while, when an electron collides with a xenon atom, the electrons in the atom will rearrange themselves, and one or more of its electrons may move into an unoccupied orbital. When the electron(s) in the xenon atom move into an unoccupied orbital, the atom is said to be in an excited state. In this excited state, the atom has more energy than it did before the collision, and this energy is stored in the rearrangement of the electrons. In other words, some of the kinetic energy of the electron that was injected into the gas is transferred to the xenon atom and this energy is stored in the configuration of the atom’s electrons. The atom, however, cannot stay in this excited state for very long; it wants to loose the energy that it gained from the collision and return to its lower energy state. When the atom decays to the lower energy state, the electron(s) rearrange themselves again and the previously unoccupied orbital will become unoccupied again. When the electrons decay out of the higher energy orbital, the atom will loose energy and emit a photon of light in the process. When current flows through the xenon gas, many of these collisions occur and many photons are emitted from the gas. The collisions of the injected electrons with the xenon atoms are therefore responsible for the flash.
Figure b): Flashlamp Flashing. When electrons are injected into the gas tube, the voltage between the plates of the capacitor drives the electrons to flow towards the positive electrode. When the electrons flow through the tube, they collide with xenon atoms, and occasionally the xenon atoms will become excited. Once in this excited state, the xenon atoms will decay back down into their lower energy levels and emit a photon of light. Since many of these light emitting collisions occur, the xenon lamp appears to flash just after the electrons are injected.
Answer: c) During each flash, the electrons injected into the gas will flow between the two electrodes and collide with xenon atoms. The xenon atoms and the injected electrons are the only thing in the gas tube, so they must somehow be responsible for damaging the electrodes, but how? Particularly energetic collisions between injected electrons and xenon atoms are capable of ionizing the xenon atoms, a process in which xenon electrons are actually ripped away from the xenon atom. When the xenon atom looses an electron, it becomes a positively charged ion, and the mixture of the positive ions and the negative electrons is referred to as a plasma. Some of these charged ions will collide with the electrodes. As it turns out, the electrodes used in the flashlamp are fragile, so it is possible that when a charged ion collides with an electrode, it can damage the electrode. When the ions hit the electrodes, they will sometimes chip away atoms from the electrode in a process called sputtering. This sputtering process is therefore responsible for damaging the electrodes.
Figure c): Sputtering. When charged ions collide with the electrode, they chip atoms away from the fragile electrode surface. This damages the electrode, and the process is called sputtering.
Answer: d) In this problem, we want to determine how the pressure of the xenon gas affects the spectrum of light that is emitted from the lamp. The first thing we need to consider is what the pressure of a gas actually means. As we know, if we increase the pressure of a gas, we are essentially adding more atoms into the gas while keeping the volume constant. This means that by increasing the pressure of the gas, the atoms in the gas become more densely packed. So how does this affect the spectrum of light emitted from the gas? Well, we know that a high-pressure gas contains many densely packed atoms, and so these atoms are colliding with each other more frequently and more violently than they would for a lower temperature gas. These collisions between atoms in the gas will actually rearrange the atomic orbitals of the atoms, which in turn will shift around the atoms’ electronic energy levels. This means that when there are more collisions, the atomic energy levels will be shifted around more. So at higher pressures, the atomic energy levels of the xenon atoms will be shifted around more than they would at lower pressures. But how does this shift in the atomic energy levels change the spectrum of light emitted from the flashlamp? The spectrum of light that comes from the flashlamp is determined by the photons that are emitted from each of the atoms. Additionally, the wavelength or color of an emitted photon is determined by the energy difference between the atom’s lower energy state and the excited state that it decays from (remember from part b that an atom is placed into an excited state when it collides with an injected electron, and a photon is emitted when the atom returns to a lower energy state). Since the atomic energy levels are shifted around more at higher pressures, the wavelengths of the emitted photons will be shifted around as well. This shifting around of the photon’s wavelength results in a broadening of the spectrum of light coming from the lamp. A high-pressure flashlamp will therefore have a more uniform spectrum of light because the spectrum is broadened by the increased number of collisions in the gas.
Figure d): Pressure Broadening. At lower pressures, the collisions between xenon atoms are less frequent and less energetic, while the collisions between xenon atoms at higher pressures are more frequent and more energetic. These collisions shift around the energies of the emitted photons, and so the spectrum of light for xenon atoms at high pressures is broadened compared to the low-pressure spectrum.
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 practical, even if you didn’t care that it was orange in color?
Answer: e) When we use the flashlamp, we want the lamp to flash on quickly for us to take a picture. So what makes a high-pressure sodium vapor lamp impractical for this type of application? Sodium is a solid at room temperature, so in order to create a gas of sodium atoms, the lamp must be heated up to evaporate the solid sodium. Since the sodium has to be heated up before is can be used, a sodium vapor lamp is not very good to use for a flashlamp, which we want to flash on quickly.