<p>Diagram of <a href="https://en.wikipedia.org/wiki/Andreev_reflection" class="extiw" title="en:Andreev reflection">Andreev reflection</a>.  An <a href="https://en.wikipedia.org/wiki/electron" class="extiw" title="en:electron">electron</a> meeting the interface between a normal conductor and a <a href="https://en.wikipedia.org/wiki/superconductor" class="extiw" title="en:superconductor">superconductor</a> produces a <a href="https://en.wikipedia.org/wiki/Cooper_pair" class="extiw" title="en:Cooper pair">Cooper pair</a> in the superconductor and a <a href="https://en.wikipedia.org/wiki/retroreflection" class="extiw" title="en:retroreflection">retroreflected</a> <a href="https://en.wikipedia.org/wiki/electron_hole" class="extiw" title="en:electron hole">electron hole</a> in the normal conductor.
</p>
Legend: "N" = normal conductor, "S" = superconductor, red = electron, green = hole.  Arrows indicate the <a href="https://en.wikipedia.org/wiki/spin_(physics)" class="extiw" title="en:spin (physics)">spin</a> band occupied by each particle.
<a href="//commons.wikimedia.org/wiki/User:Ilmari_Karonen" title="User:Ilmari Karonen">Ilmari Karonen</a>
Drawn in <a href="https://en.wikipedia.org/wiki/Inkscape" class="extiw" title="w:Inkscape">Inkscape</a> by <a href="//commons.wikimedia.org/wiki/User:Ilmari_Karonen" title="User:Ilmari Karonen">Ilmari Karonen</a>.