SPF
SPF is a number that tells you how much longer sunscreen lets you stay in the sun before your skin burns.
Sunscreen works like a stack of trays between a heat lamp and a block of ice. With no trays, the lamp melts the ice fast. Add one tray, and the ice lasts twice as long. SPF 30 means your skin gets 30 trays of shade before the burn starts.
Explaining spf by grade level
Sunscreen is like a shield for your skin. A higher SPF number means the shield is stronger. If you spread sunscreen on one arm and leave the other bare, the bare arm turns red first. A thicker layer of sunscreen can also help the shield last longer.
Projects that explore spf
SPF tells you how much longer sunscreen lets you stay in the sun before burning. This project tests that idea by exposing light-sensitive paper under sunblocks of different SPF ratings. The paper turns lighter where more UV light is blocked, so a higher SPF should give a lighter patch.
SPF is a number that tells you how much longer sunscreen lets you stay in the sun before your skin burns. This project checks that number directly by coating glass with sunscreens of different SPF values. A UV meter under each sheet measures how much sunlight gets through, so a higher SPF should let less UV pass.