In this experiment, linear polyynes are synthesized and then the predictions of a one-dimensional, particle in a box are used to calculate the quantum mechanical box length for the polyynes. A solution of graphite in ethanol is irradiated with a Nd:YAG laser (532 nm) and the resulting solution is filtered and analyzed. Data from gas chromatography-mass spectrometry shows that linear polyynes, C[subscript 6]H[subscript 2] and C[subscript 8]H[subscript 2], are produced. The absorbance spectrum of the reaction mixture is obtained and the wavelengths are used to calculate the length of the conjugated pi system for each polyyne. The calculated box lengths for C[subscript 6]H[subscript 2] and C[subscript 8]H[subscript 2] are 9.23 A and 10.82 A, respectively. Although these values differ (47% and 22% larger) from the molecular lengths determined from the summation of bond lengths and from molecular modeling software, the experiment provides students with an opportunity to enrich their understanding of the predictions and limitations of the one-dimensional, particle in a box. The experiment is suitable for inclusion in a standard physical chemistry course or other upper-level laboratory where the principles of quantum mechanics are studied. (Contains 4 figures, 2 tables, and 1 note.)