Question

Use Beer's law to explain why fingerprints on the cuvette would change the absorbance

Answer 1

In a spectrometer, the sample solution is placed inside the cuvet, and a tiny spectrum light beam is shined through it. Some light is absorbed by the sample, and the rest is transmitted and hits the (also tiny) detector on the other side of the cuvet. The "absorbance" is basically the difference between the full spectrum of the beam, i.e., before it passes through the sample, and the detected light, i.e., after it passes through the sample.

The cuvet itself isn't supposed to absorb any light, and normally it doesn't - but if it's dirty, that's a different situation. The oil/dirt particles will scatter (change the angle of travel of), and possibly absorb, some of the light. This will give a false reading.

Fingerprints will reduce the amount of light that can pass through the cuvette. As a result, transmittance will be lower and absorbance will be higher.