There are several ways to quantitate DNA and the choice of methods often depends on the amount of sample you have.
Biological molecules absorb light at specific wavelengths of the electromagnetic spectrum.
Proteins, for comparison, absorb maximally at 280 nm. We assess this features by analyzing a small diluted volume of our preparation against a blank. As you know from pigment analysis, the spectrophotometer can be programmed to shoot different spectra of light. The spectrophotometer can also measure how much light comes out of the sample and calculate how much light was absorbed vs. transmitted. From this graph, you need to trace, record, and print the Abs. values for 260 and for 280 and use these values to calculate concentration and estimate sample purity, as described in the next two sections.