A neon lamp consists of a glass tube filled with low-pressure neon gas and an electrode at each end.
1. A high voltage transformer puts lots of charge on the electrodes at either end of the tube. How is the discharge started?
Answer: A corona discharge forms.
Why: The charges piled up on the end of the electrodes repel each other. If enough charges are placed there, this repulsion will become strong enough to push some of them right off the end of the electrode.
2. After the discharge is started, electrons can collide with neon atom. Describe the three types of collisions that can occur.
Answer: 1. The electron can bounce off of the atom without losing energy. 2. The electron can lose lots of kinetic energy and leave the atom in an excited state. 3. The electron can lose lots of kinetic energy and knock an electron off of the neon atom, ionizing it.
Why: The second type of collision leaves the atom in an excited state which can then release its energy as light in making a transition to a lower energy state. The third type of collision creates the plasma through which the current flows.
3. In the discharge there are both positive ions and negative electrons. The lightweight electrons carry most of the current, but ions are necessary to keep the discharge going. What would happen to a ball of electrons created at the negative electrode as it traveled toward the positive electrode if there were no ions present?
Answer: The electrons would repel one another causing the ball to expand.
Why: For a neutral plasma each charge has as many positive neighbors as negative neighbors so there is no net force from these neighboring charges.
4. The neon atoms in the discharge mostly emit red light. In order to produce a more white light you might consider coating the inside of the tube with phosphors that emit blue light. Why won't these phosphors convert the red light emitted from the neon atoms into blue light?
Answer: Blue photons have more energy than red photons.
Why: The phosphor cannot release more energy than it is supplied with.
A neodymium:YAG laser (YAG is short for the crystal yttrium aluminum garnet) is typically optically pumped and produces infrared light at 1064 nanometers.
5. Often the pump light is produced by a high-pressure flash lamp, which is a discharge lamp that is pulsed on for a very brief time. These lamps produce a broad spectrum of visible light. Explain why any infrared light, with a wavelength longer than 1064 nm, does not contribute to pumping of the neodymium ions.
Answer: An infrared photon with a wavelength of more than 1064 nm does not carry enough energy to pump the neodymium ions to the state from which they can emit a 1064 nm photon.
6. Neodymium ions absorb only a few discrete frequencies of visible light. Recently it has become popular to use diode lasers as a source of pump light. The diode lasers are designed to emit a narrow band of colors near 808 nm, which matches one of the strongest absorption lines of the neodymium ions. Compare the efficiency of diode laser pumping to flash lamp pumping.
Answer: Pumping with diode lasers is more efficient than pumping with flash lamps.
Why: Any light from the flash lamp that does not have a frequency matching one of the discrete absorption lines of the neodymium ions will simply be wasted. In contrast, most of the photons from the diode laser can transfer their energy to the neodymium ions providing very efficient pumping.
7. Often it is desirable to produce visible light with the neodymium:YAG laser. This can be done by sending the infrared light pulse at 1064 nm through a special crystal. The crystal can combine a pair of infrared photons into a single green photon at 532 nm. Compare the energy of an infrared photon that enters the crystal to the energy of a green photon that exits the crystal.
Answer: A green photon at 532 nm carries twice as much energy as an infrared photon at 1064 nm.
Why: Since two infrared photons were used to create the green photon, energy is conserved.
8. If you shine the green light from this laser onto a thin soap film, what pattern of light would you observe reflected from the film. Assume that the film is suspended on a loop of wire that is oriented vertically (the loop is in a vertical plane).
Answer: A series of horizontal bands of green light.
Why: Interference between the front and back surfaces of the film can cause no light to be reflected (destructive interference) or lots of light to be reflected (constructive interference). The films thickness varies from thin on top to thick on the bottom because of gravity.