Shielding Ourselves from Ultraviolet Light

Transcription

1 Shielding Ourselves from Ultraviolet Light Introduction The purpose of this experiment is to examine the effect of passing ultraviolet (UV) light through a variety of materials to determine how well each material protects us from UV light. You will be given a few beads which change color (from white to red) when exposed to UV light. We will use portable UV lamps to expose the beads to both long wave (365 nm) and short wave (254 nm) ultraviolet light. The beads will fade back to white once the source of ultraviolet light is turned off. Materials Needed: - UV-sensitive beads - 50 ml beaker ml beaker - Sunscreens of various SPF (e.g. SPF-15, SPF-30, SPF-50) - Water - Dental floss - Metal washer - Squares of cotton fabric (about 2 x 2 ) - Plastic wrap - The lens from a pair of safety glasses - UV lamps that project 365 nm and 254 nm light, with shields - UV lamp stands General cautions: Do not look directly into the UV lamp. These lamps emit harmful ultraviolet rays that can permanently damage your eyes. Wear your safety glasses at all times. Do not expose your skin to the ultraviolet light turn the lamps off while you are setting up each part of this experiment. Section I: Long wave UV light (365 nm): Part A: Sunscreens Set up your UV lamp on the stand such that the long wavelength light is projected (i.e. the shield is over the short wave window). Invert a 50 ml beaker and place it underneath the long wave window. Coat beads with sunscreens of various SPFs (each bead should have a different sunscreen on it). When you coat the beads, begin with the lowest SPF sunscreen and spread the sunscreen over the surface of a bead. The coating should be even, and there should not be and globs of sunscreen on the surface of the bead. Coat one bead with each available sunscreen. After coating each bead, thoroughly wipe your hands/fingers to make sure no sunscreen is left on your hands. Place the coated beads on top of the 50 ml beaker. Also place an uncoated bead on top of the beaker as a control (Fig 1). Turn on the UV lamp and expose the beads to the long wave light for five minutes. Record your observations.

2 Fig. 1 Part B: Varying distance Turn off the UV lamp and allow the beads to return to their white color. Remove the sunscreen-coated beads from the 50 ml beaker (leaving the untreated bead) and place another untreated bead on the base of the stand (see Fig. 2). Turn on the lamp and expose the beads to the long wave light for five minutes. Record your observations and note any differences due to the change in distance from Part A. Turn off the lamp once you have finished making your observations. Fig. 2

3 Part C: Shielding For the following series of tests, you will place a 150 ml beaker right-side up beneath the long wave window of the UV lamp. Inside the 150 ml beaker, place an inverted 50 ml beaker, and place a bead on top of the 50 ml beaker. Make sure the bead has faded to white before starting each test. 1. Plastic wrap: Cover the top of the 150 ml beaker with a piece of plastic wrap (see Fig. 3). Turn on the lamp and expose the bead to the long wave light for five minutes. Record your observations, and compare the color change to that of the beads in Part A. 2. Cotton: Turn off the lamp and allow the bead to return to white. Remove the plastic wrap and place a square of cotton fabric over the top of the 150 ml beaker (Fig. 3). Turn on the lamp and expose the bead to the long wave light for five minutes. Record your observations, and compare the color change to that of the beads in Part A. 3. Safety Glasses: : Turn off the lamp and allow the bead to return to white. Remove the cotton and place a lens from a broken pair of safety glasses (Do not use the safety glasses you are supposed to be wearing!) over the top of the 150 ml beaker (Fig. 3). Turn on the lamp and expose the bead to the long wave light for five minutes. Record your observations, and compare the color change to that of the beads in Part A. Fig Glass: Turn off the lamp, dry the bead, and allow the bead to return to white. Empty the 150 ml beaker. Invert the 50 ml beaker and place it on the base of the lamp stand. Put the bead on top of the beaker. Invert the 150 ml beaker and place it over the 50 ml beaker and the bead (Fig. 4). Turn on the lamp and expose the bead to the long wave light for five minutes. Record your observations, and compare the color change to that of the beads in Part A.

4 Fig Water: Turn off the lamp and allow the bead to return to white. Remove the cotton fabric and use a squirt bottle to fill the 150 ml beaker with water. Tie one end of a piece of dental floss to a metal washer and the other end to a bead. Place the bead, floss and washer in the 150 ml beaker as shown in Fig. 5. The bead must be beneath the surface of the water and the distance between the lamp and the bead should be about the same as it was in the other shielding experiments. Turn on the lamp and expose the bead to the long wave light for five minutes. Record your observations, and compare the color change to that of the beads in Part A. Section II: Short wave UV light (254 nm): Fig. 5 Set up the lamp on the stand to project short wave light by moving the shield over the long wave window. Repeat all of the above experiments using short wave light. Record all of your observations.

5 The Report For your lab report, summarize your observations in clearly labeled tables. State your conclusions regarding how well the materials used shield the beads from UV light, and answer the following questions: Questions to be answered at the end of the experiment 1. What was the purpose of the control bead in Part A? 2. Calculate the frequency of the short wave and long wave UV light used in this experiment. Are these wavelengths in the UV-A, UV-B, or UV-C range of the spectrum? 3. Why might a material protect better against short wave light than long wave light? 4. Compare the protection (i.e. intensity of the color change) given by water, glass, the safety glasses, and cotton to the protection given by the sunscreens. What is the approximate SPF of each material vs. long wave UV light? What is the approximate SPF of each material vs. short wave light? 5. Compare the results of Part B for both long wave and short wave UV light. Does varying the distance have a more pronounced effect for the long wave or the short wave? Discuss. 6. If you take your beads outside on a cloudy day, would you expect to see the beads change color? Why or why not? 7. Glass, plastic wrap, the plastic safety gasses lens, and water are all clear they allow visible light to pass through. If a clear material does not allow UV light through, what happens to the light that doesn t make it through?