  A photoelastic material will reveal its internal stresses when observed
through polarizing filters. This eye-catching property has enlightened our
understanding of granular materials for over half a century, whether in the
service of art, education, or scientific research. In this review article in
honor of Robert Behringer, we highlight both his pioneering use of the method
in physics research, and its reach into the public sphere through museum
exhibits and outreach programs. We aim to provide clear protocols for artists,
exhibit-designers, educators, and scientists to use in their own endeavors. It
is our hope that this will build awareness about the ubiquitous presence of
granular matter in our lives, enlighten its puzzling behavior, and promote
conversations about its importance in environmental and industrial contexts. To
aid in this endeavor, this paper also serves as a front door to a detailed wiki
containing open, community-curated guidance on putting these methods into
practice.
