Experiment (2):

Detection & Absorption UV Light

OBJECTIVES

To understand what ultraviolet (UV) light is
To understand the different types of UV light
To test the abilities of materials to absorb UV light
To understand relationship between UV light and sun protection factor (SPF)

MATERIALS & EQUPMENT

UV-sensitive beads, sunscreen lotions with various SPF ratings, sunblock, sunglasses, clear and opaque plastic, glass plate, foil, cloth, small plastic bags, laboratory UV light, UV intensity meter card

PROCEDURE

Part A: Detecting UV light

Place 3-5 UV-sensitive beads in two small plastic bags. These beads will turn color in the presence of UV light. The higher the intensity of UV light the stronger the color change.
Make two bags: one labeled “Control” and one labeled “Experiment”. Record the color of the beads when exposed to indoor light (lighting in the lab). The control bag will remain at your bench, and should be used for comparison to color changes in experiment.
Take the experiment bag outside, in an area of direct sunlight. Record the color of the beads after 10 seconds of exposure to outside sunlight.
Find a shaded area (e.g. under a tree or in a corridor). Record the color of the beads in shade.

Part B: Absorption of Outdoor UV light by various materials

Place on a tray: control and experiment bags from Part A and one of each of the materials from the front bench (clear plastic, opaque plastic, sunglasses, glass plate, foil and cloth). Take the tray outside to an area of direct sunlight.
Use each type of material to block the sunlight by holding directly above the experiment bag. Record the color of the beads when shielded by each sample. Note the colors of both control and experiment beads.

Part C: Absorption of UV Light by lotions

Place 3-5 UV-sensitive beads into a small plastic bag; make four bags. Label with the SPF numbers, and one sunblock. Note the color of the beads under indoor lighting.
Add one small drop/dab of each lotion onto each bag. Lightly coat/spread to cover one side of the bag. Allow lotion to dry. Bring the bags to a laboratory UV light setup. Leave bag for 10 seconds. Look into the front of the setup (but not directly at the light) to record the color of the beads under UV light.
Empty the beads into beaker at front bench. Discard coated plastic bags.

Part D: UV Light at SMC

1. Obtain a UV Intensity Meter card from the front bench. Go outside to the same area of sunlight you had been working in previously. Record the reading on your report sheet.

EXPERIMENTAL DATA

QUESTIONS

What is the difference between UV-A, UV-B, and UV-C light?

Compare the energies of UV light to IR and visible light. Explain why UV light is potentially more dangerous than IR or visible light.

Opacity is the opposite of translucent (clear); for a completely opaque substance, you cannot see through it at all. Considering results from the plastics, glass, foil, and cloth pieces, did the opacity of each substance affect ability to block UV light? How might you use this knowledge to protect yourself when outdoors in sunlight?

What is the purpose of an experimental control? Give one specific example of a control used in this experiment.

Most sunscreen lotions claim to protect against UV-A and UV-B. Why don’t they mention UV-C light?

From your data in Part C, do higher SPF ratings provide more protection from UV light?
If the L.A. Times reports a UV Index of 6.5, and taking into account your skin pigmentation, how might this affect your plans for the day?

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