Sunscreen blocks UV rays from the Sun that could cause long-term damage to our skin, including cancer. The Earth's atmosphere blocks a lot of these rays, but some of them can make it through and that's why we need to put on sunscreen. Sunscreen is formulated to block the wavelengths of light that can be harmful. Astronomical telescopes use a similar kind of method to make sure that they only see the specific wavelengths of light that they want - they put on coatings that block certain wavelengths and allow others through!