Experiment of the Month

Speed of light.

The MU Physics Department does not claim that we invented these labs. Actually, the origin of these labs are currently unknown to us. Our labs do not have written instructions. In keeping with this spirit, the description given here will be brief and general. The intent is that each performance of the lab will be unique; in each nature will reveal a slightly different face to the observer.

This month's lab requires an excimer laser, which produces 3 nanosecond duration light pulses with pulse energy of about one Joule. This device is not available for introductory labs, and so is not in the same category as our usual featured experiments. The work described here was done by senior physics major Brian Portock over the course of two semesters. He used the project for both his Departmental Honors program and his senior seminar presentation.

The goals of the project may be divided into short-term, near-term, and long-term. The short term goal was to construct the safety tunnel shown in the sketch below. The intensity of the laser is enough to melt glass when the light is focused to a spot. The lids and doors in the tunnel allow construction of experimental apparatus inside. They are sealed against light leakage so that experiments may be carried out with reduced concern for stray laser light.

figure 3

The near-term goal was to measure the speed of light in air, which is nearly the same as the speed in vacuum. Mirrors were placed at the two corners of the tunnel to direct the laser light pulse as it travelled to the final mirror and returned to a detector. The laser light path is indicated in the figure above with a long arrow. On the right side of the sketch is a 2 meter length of PVC pipe, with mirrors mounted in both ends. Light enters the pipe through an un-silvered part of the mirror, and can reflect several times as sketched below.

figure 3a

After reflecting in the pipe, light leaves through a non-silvered section of the farther mirror, and reflects in "corner cube" which sends it back in just the opposite direction. The (slightly offset) beam retraces its path and finds its way to the detector.

Light travels about 1 meter in 3 nanoseconds. The path traveled by the light varied from about 12 to about 18 meters, depending on the number of reflections in the PVC pipe. By comparing the times for 1,2, and 3 round trips in the pipe, Brian measured the speed of light to be 3.0 +/- 0.1 x 10^8 m/sec.

The long term goal was to determine the difference between the group velocity of light and the phase velocity of light, traveling in water. A calculation based on published data for the phase velocity as a function of wavelength indicates that the group velocity will be about 2% slower that the phase velocity. When the PVC pipe was filled with water, the observed velocity did indeed decrease. At present, however, the measurement accuracy needs to be improved by a factor of five in order to determine with confidence the difference between the phase velocity and the group velocity. Since Brian Portock is moving on to graduate school in engineering, this improvement awaits the interest of another senior physics major.