Science Buddies: "Ask an Expert"

Good Evening,

My daughter is in 7th grade and is having problems with the above topic. All of the spf's are coming out with a UV index of zero. We used the procedure listed on the website. I emailed teacher and she is ok with project failing. I was just wondering if we did something wrong. We would get a control number of 7 . Put the sunscreen on the plastic wrap do another reading. It was always 0. For all spf's. My second question is subrtacting the control from the sunscreen reading. If that is 0 - 7 then that equals -7. I cant wraap my head around why it is subtracted that way and why? What is that number telling me. I am trying to help her understand but I am not having any luck myself. Any help you could provide would be great. Thanks!

Kim Bateman
stressed mom

The experiment procedure is a bit short on detail. Really all that subtraction is telling you is the number of UV indexes that are blocked by the sunscreens... it doesn't really matter whether these are expressed as positive or negative. A negative number would be interpreted as "this is how much the sunscreen changed the reading by"... in other words, -7 would mean the index went down by 7 due to the presence of the sunblock.

As for why you're getting 0 for all your readings, a few things are possible. I'd say the most likely is that the layer of sunscreen was too thick. You might try rubbing it onto the plastic like you would rub it into skin until the layer is thin enough that you can't see it very well. That will also help cut down the variability between the samples. Another possibility is that you didn't have enough sunlight during your test. I'm not sure how your meter is calibrated, so it's hard for me to tell if "7" is high or low.

Thank you for your reply, I will have her try what you suggested. Maybe we will choose a fabric or cheese cloth next time. The teacher doesn't want her to change anything at this point so we will stick with what we have. Thanks for explaining the subtraction, I thought that's what it was just needed someone else to confirm. Thanks Again for your help.

Kim Bateman