Students determine the amount of cranberry juice (as a volume percent) in mixtures of cranberry juice and apple juice by measuring the absorption of mixtures and comparing to a calibration curve prepared for cranberry juice. The experiment assumes that the absorbance of apple juice is negligible at the analytical wavelength, which isn’t strictly true. The experiment can be modified easily by a simultaneous analysis at two or more wavelengths.
The full citation is here: Edionwe, E., Villarreal, J. R., Smith, K. C. J. Chem. Educ. 2011, 88, 1410–1412 and a link to the article is provided below (subscription to the journal required).