simulación de cargas y campos muy útil

<div style="position: relative; width: 300px; height: 191px;"><a href="http://phet.colorado.edu/sims/charges-and-fields/charges-and-fields_es.html" style="text-decoration: none;" rel="external"><img src="http://phet.colorado.edu/sims/charges-and-fields/charges-and-fields-screenshot.png" alt="Cargas y Campos" style="border: none;" width="300" height="191"/><div style="position: absolute; width: 200px; height: 80px; left: 50px; top: 55px; background-color: #FFF; opacity: 0.6; filter: alpha(opacity = 60);"></div><table style="position: absolute; width: 200px; height: 80px; left: 50px; top: 55px;"><tr><td style="text-align: center; color: #000; font-size: 24px; font-family: Arial,sans-serif;">Click para iniciar</td></tr></table></a></div>