You may want to look resonance up. There are all manners of physical systems that have a natural oscillation frequency, be this mechanical, optical, or whatever. When excited by a multi-frequency signal, they will amplify their natural frequency more than any other. So in a way you can think of them as blocking all other frequencies.
OK, look at this image taken from the wikipedia article... Lets say you have a system with a resonant frequency of 100 Hz. Any real system also has some amount of attenuation due to friction, which is indicated by the $\delta$ parameter. Lets say that our system follows the curve $\delta = 0.2\omega_0$.
So lets now excite this system with a combination of three frequencies: 50, 100 and 200 Hz. The 50Hz excitation will be amplified to about 133% of the input, the 100 Hz excitation to about 240%, and the 200 Hz to only 33% of the input.
As the graph shows, you can actually use any resonant system with strong dampening to filter out the higher frequencies, and adjust the cut frequency with the natural frequency of your system.