Lasers and LDR's

[DavidSh]

If not, is there a lens that only lets a specific frequency through?

There are narrow-band filters called Dichroic Filters that can be bought. It looks like the narrower the band you need, the higher the price. For example, a one-inch diameter filter that passes green light of wavelength between 505 nm and 575 nm goes for about $20. (That's nanometers, not nautical miles.)

But I thought the topic was going to be a new RPG, "Lasers and Long-Distance-Relationships".