Consider a cw laser consisting of two mirrors (r, - 100%, r2-99%, the distance between mirrors l) and a laser element pumped by a diode laser or a flash-lamp. how the pumping power threshold will change if (i) r, is reduced from 99% to 95%, (ii) the emission cross section is increased from to 2 (ii) the life-time of the metastable level is increased from to 20% (assume four-level laser scheme.)

3. dark matter

empty space could also be a simple vacuum

chemical energy --> gravitational potential energy --> kinetic energy --> elastic potential energy --> gravitational potential energy

explanation:

when emma starts the hike, the chemical energy stored in her body is used to climb up the mountain, and so she gains gravitational potential energy (because at the end, she will be at a higher height h above the ground). then she goes bungee jumping: as she falls, this gravitational potential energy is converted into kinetic energy (because her speed increases as she falls down), until the point when the rope stretches, and she stops moving. at that moment, all the kinetic energy is converted into elastic potential energy due to the stretching of the rope. then, this energy is released because the rope returns to its natural length, and emma gains a small increase in altitude , so she gains some gravitational potential energy.