Ultra violet radiation can be damaging to skin, leading in the worst case to melanoma and possible death. As aconsequence, we normally rub sunscreen oun our skin before going out in bright sunlight. Sunscreens are liquids containing chemicals that will block the dangerous uv radiation (or at least a portion of it) from reaching our skin.
The effectiveness of these sunscreens has been followed for several years using uv beads. These are small plastic beans with a uv sensitive dye indorporated in them. On exposure to uv light, they change colour. However, there must be others out there apart from me who have hankered after some ‘proper’ wet chemistry to allow the same sort of investigation.
The experiment here uses a version of the cyanotype photographig process: you have a yellow solution that changes fairly rapidly to blue on exposure to uv light. The real advantage of this over the uv beads is that you can use a colorimeter to take measurements and thus get quantitative data rather than just qualitative.