Achromat; 1 - crown glass, 2 - flint glass, 3 - focal point for green light; 4 - common focal point for red and blue light. Note: refraction angles are axaggerated and may not reflect true run of light beam.
<a href="//commons.wikimedia.org/wiki/User:Pko" title="User:Pko">Pko</a>
<span class="int-own-work" lang="en">Own work</span>