Your body, and even your DNA, are under constant assault from ultraviolet light. Sometimes that's just an unpleasant fact of life. Sometimes, though, the hyperchromic effect and UV light help scientists know when there's a problem - like when DNA has been destroyed.
The energy from the sun gives us the energy we need to maintain most of the life on Earth. But because the sun is the only game in town, it takes advantage of its monopoly on power to torture us. Ultraviolet light does a lot of damage to us, mostly to random proteins in our skin, but sometimes to our actual DNA. When it does – when almost anything does – UV light also serves as a tattletale, letting us know how much of our DNA is damaged.
When light hits different objects it doesn't always have the same effect. In order to absorb a specific type of light, the object needs a specific structure. For example, x-ray films show off our skeletons because x-rays can best be absorbed by the calcium in our bones, and generally move right through our flesh. DNA should be an ideal structure for absorbing ultraviolet radiation. Each strand contains nucleic acids, which are just the right size and composition to absorb UV light. This is bad news for every living thing, since high-energy light is not the kind of thing that you want randomly screwing around with your nucleotides. Fortunately, when two strands get close together – like in a double helix – they alter their structure just enough to severely reduce absorption of UV light.
DNA strands aren’t always together, though. When the DNA is denatured, either by overheating or some other process that causes it to unravel, its strands separate enough to drastically increase the UV absorption in the material. Under UV light, healthy DNA is light. As it becomes useless and denatured, turns dark as it absorbs more and more radiation. Scientists call this the hyperchromic effect. It's useful in experiments, allowing scientists to check exactly how much damage is done to DNA by different processes. All they need is a black light (which surely they must have in the lab for late night raves) and some spectrographic analysis.