Laser Reflection

When you turn on a flashlight, how long does it take for the beam of light to reach the opposite wall? Not very long! It has always been clear that light moves quickly, but exactly how fast is difficult to measure. One of the earliest instruments used to measure the speed of light was the Fizeau-Foucault apparatus, named after Hippolyte Fizeau and Leon Foucault. This apparatus used a powerful light source, a distant mirror, and either a spinning gear or rotating mirror to measure the time lapse from light emission to when the reflected light returned. In 1862, Foucault obtained a value within 1% of the modern value. This method is often recreated in college physics classes using lasers.

Students can experiment with lasers and mirrors using the Laser Reflection Gizmo. In the Gizmo, students bounce a laser off an angled mirror and measure the angle of incidence and angle of reflection. Once they have established how these angles are related to one another and to the angle of the mirror, students can use the laser to investigate a “mystery mirror” that has a wavy surface. By moving the laser up and down, students can map the surface of each mystery mirror.

The speed of light has a prominent place in the history of science. It is critical for determining the distances to faraway stars and other galaxies. The constancy of the speed of light forms the basis of Einstein’s theory of special relativity.