Speed of Light

Introduction

The velocity of light is one of the most important and intriguing constants of nature. Whether the light comes from a laser on a desk top or from a distant star, the speed of light is constant. The speed of light is also important for other reasons. It establishes an upper limit to the speed of any object, according to Einstein’s special theory of relativity, and objects moving near the speed of light follow physical laws which are drastically different from Newton’s laws. Some of the most accurate early measurements of the speed of light were those made by Albert Michelson between 1926 and 1929, using methods similar to those employed here.

Equipment

Pasco OS-9262 Basic Speed of Light Apparatus, which includes:

· Instruction manual and experiment guide

· High speed rotating mirror assembly

· Fixed mirror

· Measuring microscope

· He-Ne laser

· 1 m optics bench

· Bench coupler

· Lenses

· Calibrated polarizers

· Component holders

· Alignment jigs

Procedure

First you must set up and align the equipment. Follow the instructions in the Pasco manual for this. Then, measure the speed of light. Take at least 10 measurements. Determine the speed of light and compare it to the standard value. Discuss the accuracy you can expect from this apparatus (i.e., identify and quantify the sources of error). You should be able to determine the speed of light to within a few percent of the accepted value. Be sure to include all sources of error in your analysis.

Alignment Tips

Before placing the lenses and microscope in their proper locations, use the alignment jigs to align the laser and the rotating mirror. Use the levelling screws on the optical bench and alignment jigs – you want the laser beam to be as parallel as possible to the optical bench. Set the rotating mirror so that the incident laser beam gets reflected back through the jigs. Use paper shims if necessary between the rotating mirror and the optical bench.

When everything is finally in place, the beam observed with the microscope will appear as a broad spot in a field illuminated by scattered laser light from the first lens. Check to see that this spot disappears when the light between the rotating mirror and the fixed mirror is eclipsed.

When the rotating mirror is turned on, a faint stripe will be observed along with a fuzzy spot which moves when the rotation rate is changed. If you don’t see this, something is wrong! The fuzzy spot should disappear when the beam between the fixed and rotating mirrors is eclipsed.