Every time you post, we get a clearer picture of your knowledge level. The term "decibel" is not restricted to measuring sound levels and is often used for electrical, radio and optical power.
Decibel: 1. A dimensionless measure of the ratio of two powers, equal to 10 times the logarithm to the base 10 of the ratio of two powers P1/P2.
It means that as R grows the attenuation grows exponentially
Yes and you can use that formula to calculate the amount of attenuation. It isn't much at all in clear air. He used an attenuation coefficient between clear air and haze just to be fair. He calculated that a laser beam would be attenuated to 40% of it's original strength over five miles.