a. A pulsed ruby laser emits light at 694.3 nanometer .Consider an 11.7 ps pulse containing 7.38 J of energy. Compute the physical length of the pulse as it travels through space. Planck's constant is 6.626 × 10-34 J • s and the speed of light is 3× 108 m/s.
Answer in units of mm.
b. Calculate the number of photons in it.
c. If the beam has a circular cross section of 0.316 centimeter diameter, calculate the number of photons per cubic millimeter.
Answer in unit's of mm-3.