Using the Apache Point Observatory Lunar Laser-Ranging Operation (Fig. P23.61, page 822), the distance to the Moon is determined by how long it takes for a laser pulse to travel from the Earth to the Moon and then reflect back to the Earth from special reflector arrays placed there by the Apollo astronauts. A 2.3-W laser sends 20 pulses per second (each only 100 picoseconds long) at a wavelength of 532 nm. Due to atmospheric absorption and dispersion of the beam, only a very faint reflected pulse arrives back at the observatory with a power of 3.0x10^-17 W. 

(a) How many photons does the observatory send in an outgoing pulse? 

(b) How many photons are received in a returning pulse? 

(c) How many photons are sent for every one that is detected upon return?

Figure P23.61