A Fabry-Perot laser cavity is made of two identical mirrors. Each mirror has an optical power reflectance R=0.999. The distance between the two mirrors is d=10 cm. The laser cavity is filled a gas material as the gain medium with an index of refraction n=1.05. (a) What is the photon life time of this laser cavity? (b) In order to make a laser using this cavity, you need to pump the gain medium to have a gain for compensating the losses at two mirrors. What is the threshold gain coefficient for this laser?