This website is part of gwoptics.org, which was developed by LIGO scientists in Birmingham, UK. You will only be able to run this activity using Firefox browser, and you must first ensure that you have the updated Java plugin, See the instructions linked above in Section 2) for how to update the Java on your local machine (either Windows or Mac).
In this applet you can tune a simulated Fabry-Perot cavity within a Michelson interferometer and see how your tuning changes the power output of the device. One of the cavity mirrors is curved, while the other is flat. The applet allows adjustments to the position and curvature of the end mirror, and the reflectivity of both mirrors. Try to determine the conditions that maximize the power output.