Carey Fisher wrote: > On Thu, Jun 23, 2011 at 9:31 AM, Michael Watterson wro= te: > > =20 >> On 23/06/2011 13:15, Yigit Turgut wrote: >> =20 >>> Intensity of light is reduced by inversely proportional to >>> square of the distance. >>> =20 >> not true with a laser. That would imply the spot doubles in diameter >> with double distance. many don't >> -- >> >> >> The Inverse Square Law only applies to a point (or equivalent) source >> =20 > radiating energy in "all" directions - "all" directions meaning a spheric= al > surface subtending an angle of 4pi steradians. > Carey > =20 Correct me if I'm wrong, but here is my understanding of it: Any beam of electromagnetic waves must be diverging or converging, as it=20 would be physically impossible to make them exactly parallel. If the=20 energy density is measured at distance d from the focal point, and also=20 at distance 2d from the focal point, the energy density at 2d will be=20 1/4 the energy density at d. At 3d, it will be 1/9 what it is at d. The inverse square law applies, you just need to measure from the right=20 starting point. Kerry --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .