Suppose we had a laser system with a stimulated emission cross section of 10-16 cm2 and an initial inversion of 1014 cm-3 filling a cavity 50 cm long. the cavity uses mirrors with reflectivities r2 = 0.9 (t2 = 0.1) and rl =0.98 (t1 = 0) and contains a nonsaturable loss of 2% per roundtrip. (a) by what factor is the system above threshold (i. e., what is the ratio of δn/δnt)? (b) estimate how long it would take for the laser intensity to increase (from noise) by a factor of 105. (ignore saturation.)

Which of the following correctly describes what happens when an atomic bomb explodes? small pieces of fissionable material are joined and form a body with a mass greater than the critical mass, the relative number of neutrons escaping decreases, and a chain reaction and explosion result. large pieces of fissionable matter are brought together quickly and form a body with a mass smaller than the critical mass, the relative number of escaping neutrons increases, and a chain reaction and explosion result.