Visualizing and manipulating the spatial and temporal coherence of light with an adjustable light source in an undergraduate experiment

Coherence expresses the ability of light to form interference patterns stationary in time and extended over a spatial domain. The importance of coherence is undoubted but teaching coherence constitutes a challenge. In particular, there are only a few simple and clear experiments to illustrate coherence. To render the phenomena of coherence more accessible and to point out the difference between spatial and temporal coherence, we introduce an undergraduate experiment consisting of a light source illuminating a double-slit and a Michelson interferometer. The light source is adjustable in its spatial extent, its central wavelength and its spectral width. With that we can control the spatial and temporal coherence of the emitted light. The variation of the spatial and temporal coherence causes changes in the contrast of the interference patterns in both interferometers captured simultaneously by two cameras at the output of the double-slit and the Michelson interferometer. Therefore, the change in spatial and temporal coherence can be directly visualised.