pervect

I don't think the opacity is varying per se.

I believe the situation is that a plasma will reflect waves of low frequency, and transmit waves of high frequency.

The frequency at which this transition occurs (reflection vs transmission) depends on the characteristics of the plasma - essentially it's electron density. Thus our ionosphere reflects radio waves, but transmits light. A denser plasma will thus reflect light as well as radio waves, which is apparently what the laser people are taking advantage of.

See for instance:

http://image.gsfc.nasa.gov/poetry/ask/a10013.html

I think the technical term for this frequency is "plasma frequency", but I'm really not positive.

You might also try talking about this topic on rec.arts.sf.science (that's a usenet newsgroup).