need expresion to change opacity

Help,

I'm creating a radar for a class project and I'd like the opacity of my "plane" layer to change whenever the radar "sweep" makes contact.  I guess that would be a collision detection?  Basically, I'd like to have the opacity of the "plane" layer dark, around 50%, but then jump to 100% whenever the radar "sweep" makes contact and then fade back to the original value.  Any suggestions?

Thanks,