No abstract

The availability of low cost digital multimeters capable of measuring capacitance has made parallel plate capacitor investigations common in the introductory laboratory. Typically, students add multiple thin sheets of dielectric material between conducting plates. The capacitance is measured and then plotted versus the reciprocal of the dielectric thickness ͑the nominal plate separation͒. We explain why the experiment fails for small dielectric thickness and discuss an improved version of the experiment using dielectric sheets of various thicknesses rather than multiple thin sheets of a dielectric.

No abstract

We report on spectroscopic measurements of the Rb SD,,, state performed using two-photon excitation with a resonant intermediate state. With the two beams (5S,/, -+ 5P,/, at 780nm and 5P,/, + 5D,/, at 776nm) counterpropagating, the spectra are Doppler-free and the signals are very robust, even in a room temperature vapor. This is due to the near match of the transition wavelengths. The 5D,/, hyperfine constants have been measured and compared to previous determinations. We have also performed the excitation in a sample of laser-cooled atoms and compare these results to those obtained at room temperature. The fact that the 5P,/, + 5D,/, transition is narrow, robust, diode-laser accessible, and relatively immune to AC-Stark shifts, may make it a promising candidate for a frequency reference at 776 nm.

To help students develop an understanding of the proper use and function of spectrographs and monochromators we describe a student-assembled spectrograph using a ‘webcam’ detector. The apparatus also works well as a low-cost demonstration, helping students make connections between an atomic spectrum observed by eye and a plot of the relative intensity versus wavelength. It also allows students to observe the internal components of a spectrograph.