When driving on a sunny day, one can sometimes observe a small solar spectrum in the rearview mirror of a vehicle. The formation of this “rainbow” results from the prism-like structure of the rearview mirror. Our investigation of this phenomenon reveals that the resulting spectrum is formed in a manner similar to the way a rainbow is formed by a water droplet. We suggest a demonstration that you can perform so that your students can observe this phenomenon. All that is required is a sunny day, a ring stand, a clamp holder, and rearview mirror.

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