The gang's lunar laser ranging experiment utilizes a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, as is referenced on the whiteboard near Sheldon and Leonard's apartment door. Nd:YAG crystal lasers typically emit light with a wavelength of 1064 nm, in the infrared; however, the high-intensity pulses may be efficiently frequency doubled to generate green laser light at 532 nm. Both of these wavelengths are written on the lower half of the whiteboard, along with the chemical formula Y3Al5O12 for the yttrium aluminum garnet crystal structure and the atomic transition 4F3/2 → 4I11/2 in the neodymium ion, after being "pumped" into excitation. The upper half of the whiteboard calculates the round-trip travel time of a light pulse bouncing
