What effect do SPF numbers on sunblock lotions have on their effectiveness of filtering UV radiation?
You can discover this answer for yourself, but Do not use your skin to test the effectiveness of SPF numbers!
Instead, use solar active materials. I have found several materials on the Solar Active website that can be used. These include, fingernail polish, shoe strings, tread, and beads.
Think About This!
The SPF number is suppose to be an indication of how long the sunscreen is effective. To determine this in minutes, multiply the SPF by 10. For example, SPF 30 = 300 minutes (30 x 10), or 5 hours.
If SPF 15 is effective for 2 1/2 hours and SPF 30 is effective for 5 hours, does SPF give more protection during the first 2 1/2 hours than does SPF30?
The height of the Sun affects the amount of radiation received. If the Sun is really high in the sky the UV radiation received by your skin is greater than when the Sun is lower in the sky.
Clues for Investigating
(1) I suggest that you place your testing materials so that they receive direct rays from the Sun. You can use a wooden dowel to determine the angle of sunlight. Stick the dowel into the ground and point it towards the Sun. Change the angle of the dowel until the shadow it casts is as small as possible.
(2) If you lean a flat surface against the stick, direct rays from the Sun will strike the surface.
(3) Design testing material that will be used to test the effectiveness of SPF numbers. For example: Use Solar Active shoe strings, yarn, or thread. The diagram shows shoe strings threaded through holes in a piece of cardboard. Yarn or thread could be wrapped around the cardboard.
(4) Now design a container that allows sunlight to enter and hit the testing material. A shoe box would work. You could cut a section out of the top of the box and cover it with a clear transparent material, such as used for plastic report folders. Cover the plastic with a layer of sunblock.
(5) Place the testing material inside the box, and then stand the box against the dowel so that direct rays pass through the layer of sunblock and hit the testing material inside.
(6) In order to compare the effectiveness of two or more different SPF ratings, you need to prepare a testing box for each of them. You also need a Control TESTING Box, which will be exactly like the other boxes prepared except the clear plastic on the lid will not be covered with anything.
(7) All the testing boxes need to be set up at the same time so that they each receive the same amount of sunlight. You need to design a way to cover the boxes so that no light enters until you are ready for the testing to begin. The same method can be used to cover the boxes when you want the testing to STOP!
(8) Take the covered boxes indoors and out of any direct sunlight. Compare the color of the testing materials. If you do not see any difference, test the materials again for a shorter length of time. Note: Your testing materials as well as the testing boxes can be used again unless the sunblock as been wiped off.
For more information about ultraviolet light, see :
ULTRAVIOLET LIGHT (UV) INDEX