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What Blocks UVA Rays?

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Author: Randall Frost, Ph.D.

Grade Level: Middle School; Type: Physical Science

Objective:

The project compares the ability of selected materials to block UV-A radiation. The goal is to have the student formulate, test and revise a hypothesis about the ability of optically transparent materials to block UV-A radiation.

Research Questions:

  • Does UV-A radiation pass through glass and other optically transparent materials?
  • Which materials block UV-A radiation?

Ultraviolet (UV) radiation is part of the light spectrum that reaches the earth from the sun. The ultraviolet spectrum, which is invisible to the human eye, is made up of light having different wavelengths. These wavelengths are classified UV-A, UV-B, and UV-C. UV-A radiation corresponds to the longest wavelengths in the UV spectrum, and makes up about 95 percent of the UV radiation that reaches the earth.

UV-A radiation is less damaging to humans than UV-B radiation, but it is much more prevalent. Whereas UV-B radiation can be blocked by glass and some clouds, UV-A radiation can pass through them. Sunscreens may not afford adequate protection against UV-A radiation.

UV-A radiation is responsible for tanning; UV-B for sunburn. UV-A contributes to skin wrinkles, and like UV-B radiation, it can contribute to premature skin aging, eye damage, and skin cancers. The best way to get protection from UV-A radiation is to wear protective clothing. 

Materials:

  • Fluorescent black light (BL or 350BL designation);
  • Ultraviolet detecting beads (active in the UVA range);
  • Glass samples,
  • Water

Plug-in and handheld fluorescent black lights are readily available at hardware and specialty stores. They can also be purchased on the Internet, as can ultraviolet detecting beads.

Experimental Procedure:

  1. UV-A radiation is the part of the solar spectrum that is responsible for tanning. Based on your own experience with tanning, formulate a hypothesis to predict which optically transparent materials best block UV-A radiation.
  2. Locate samples of light-transmitting materials. These might include clear glass, tinted glass, eyeglass lenses, sunglass lenses, UPF-rated fabrics, and water.
  3. Observe the color of the ultraviolet detecting beads in a partially darkened room.
  4. Shine the black light source on the ultraviolet detecting beads, and note the color of the beads.
  5. Place the first sample on top of the ultraviolet detecting beads and shine the black light though the sample so that it is aimed at the detecting beads underneath. Note whether the color intensity of the beads.
  6. Rank the bead color intensity on a scale of 1 to 5, with 1 corresponding to the color of the unillumined beads and 5 being the color of the beads under the unobstructed black light.
  7. Repeat this test for each of the materials to be examined.
  8. Examine your data and note any patterns.
  9. Evaluate your hypothesis in light of your findings. If necessary, revise it and conduct more experiments to test it.

Material

Color change ranking

None (air)

5

Clear glass

 

Tinted glass

 

Eyeglass lens glass

 

Sunglasses lens glass

 

Clear plastic

 

UPF-Fabric 1

 

UPF-Fabric 2

 

Water (in clear glass)

 

Terms/Concepts:  Ultraviolet spectrum; UV-A

References: Ultraviolet Detecting Beads; Ultraviolet Radiation Indoors: What You Don't Know Can Hurt You; Sun Protection Quiz; Understanding UVA and UVB; Black Light Bulbs; Hand Held Black Light Fluorescent Device; Plugin Black Light

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