A ruby crystal rod operates as an amplifier for the radiation at 0 = 694.3 nm that coincides with the central wavelength of the ruby's atomic transition. The quantum efficiency of the transition is close to 100 %, the spontaneous emission lifetime is sp = 4.3 ms, and the transition cross section at the central wavelength is (0) = 2.5 × 10⁻²⁰ cm² . a). Assuming a homogeneously broadened Lorentzian transition lineshape, calculate the transition bandwidth Δ (FWHM).