In this, the fourth installment of Engineer Guy Series #4, Bill, Patrick, and Nick show off the engineering that accounts for three key characteristics of laser light: single wavelengths, narrow beams, and high intensities. Highlights include laser retinal surgery, the difference between phosphorescence and fluorescence, and a rather more sophisticated treatment of laser physics than is common in popular science. I’ve seen my share of “laser basics” media, before, but don’t recall any others that venture as deep as “population inversion.” [Thanks, Bill!]