A LASIK visioncorrection system uses a laser that emits 10ns

A LASIK vision-correction system uses a laser that emits 10-ns-long pulses of light, each with 2.7 mJ of energy. The laser beam is focused to a 0.87-mm-diameter circle on the cornea. What is the electric field amplitude of the light wave at the cornea?

Solution

One can determine the magnitude of the poiting vector from pulse energy and period

                                                               S = 2.7 *10^-3 J / 10* 10^-9 sec

                                                                  = 2.7 * 10 ^5 W

      the magnitude of pointing vector S = E ^2 / c u_o                 c = light velocity in vaccume , u_o = 1.25 * 10^-6 N /A^2

                                                                E = sqrt ( S * c * u_o )

                                                                    = 1.0062305 * 10^ 4 N . C^-1