The Sunscreen SPF Test

How much protection does your sunscreen give you?

There are so many different sunscreens available with different Sun Protection Factors (SPF). What SPF lotion really works to keep out the sun’s harmful UV rays? To find out, all you need is a pack of Sun Sensitive Paper, sunscreen, and sunlight.

Experiment Materials

  • Sun Sensitive Paper or UV Beads
  • Sunscreen
  • Zipper-lock bags
  • Tub with a little water in the bottom
  • A sunny location

Experiment Videos

Experiment

1

WARNING:The Sun Sensitive Paper is sensitive to sunlight and will change when placed in sunlight, so keep it in the package until you are ready to use it.

Start by collecting various strengths of sunscreen like SPF 4, SPF 15, SPF 30, and SPF 70.

2

Start out inside. Line up your sunscreen bottles and take out the first piece of paper. Label the back of the paper with which SPF you are using and then place the paper inside one of the zipper-lock bags and seal it. Smear the sunscreen on the outside of the bag.

3

Repeat this process until you have made a bag with paper for each SPF. Don’t forget a control sheet – a blank sheet of paper in a bag without any sunscreen. Then head outside.

4

Set the paper out in the sun and wait for about 5 minutes. Bring the paper back inside and submerge it in the tub of water to stop the process and fix the images on the paper.

The more blue your paper is, the more the UV rays came through. The more white the paper is means the UV rays were blocked by the sunscreen.

How Does It Work

The Sun Sensitive Paper is a great tool for this experiment because reacts to light waves and particles when exposed to ultraviolet (UV) light, due to the light-sensitive chemicals on it’s surface.  When UV Rays are blocked from the paper, the paper turns white.  However, when the paper is exposed to the UV light, it turns blue.  The darker shades of blue indicate more exposure to UV light.  Placing the paper in water stops the process of reacting to UV light and preserves the colors on the paper as they are.

How does this reaction occur? The key is in the chemicals coating the paper. Photosensitive paper is made by covering a sheet of paper with Berlin Green; a water-soluble, bluish-green compound called iron (III) hexacyanferrate (III), Fe[Fe(CN)6].  When Berlin Green is exposed to ultraviolet light, a chemical reaction takes place. The Berlin Green is changed into chemical called Prussian Blue, or iron (III) hexacyanoferrate(II), Fe[Fe4(CN)6]3, which is no longer soluble in water.

When you rinse your paper in water, the water-soluble Berlin Green washes away, but the water-insoluble Prussian Blue stays on the paper. The shade of the Prussian Blue depends on how much UV light the paper was exposed to.  Factors such as the amount of time the paper is exposed to the light source and the intensity of the light source can change the shade. For example, Sun Sensitive Paper works much better on a sunny day than on an overcast day.

Science Fair Connection

Testing the effectiveness of different sunscreens is a great science fair project, but many other items can also protect us from the harmful UV light in the sun.  You can create a new science fair project by identifying different variables, or something that changes, in this experiment. Let’s take a look at some of the variable options that might work:

  • Try testing other methods of sun protection such as cooking oil, tanning lotion, or baby oil. Which is most effective?
  • Try testing the sunscreens in a tanning bed. Are they still as effective as they were in the sun?
  • Try leaving the sunscreen bags out for different amounts of time.  For example, is SPF 15 as effective after 2 hours as it is after only 20 minutes? How long are different levels of sunscreen effective?
  • Try covering the paper with different fabrics, such as swimsuits or swim shirts that claim to have SPF protection.   Slide a piece of paper inside the suit and leave it in the sun. How does this protection compare to the sunscreen?
  • Try testing different make-ups or lotions that claim to have SPF protection.  How do these compare to the sunscreens or fabrics?

Those are just a few ideas, but you aren’t limited to them! Try coming up with different ideas of variables and give them a try. Remember, you can only change one thing at a time. If you are testing different fabrics, make sure that the other factors remain  the same!