Invisible UV radiation forms a distinctive blue color due to the cyano- type process. Sun cream, cotton, or sun-tanned skin prevents this process and the filter paper remains virtually yellow-green.
Part C: Sunscreen Through Absorption This experiment is concerned with the protection mechanisms of plants in strong sunlight. Research task: Not only do people need to be protected
from UV radiation, plants also require sunscreen. For example, cacti in the highlands are covered in dense hair which enables them to reflect part of the UV radiation. The horse chestnut, ash, and narra trees or common orange lichen have a differ- ent strategy. This experiment shows how UV-active substances absorb sunlight. (Tip: The horse chestnut contains a water- soluble, UV-active substance between its bark and wood.) Materials: 1 cyanotype filter paper from experiment A,
scissors, 2 snap capped vials with a seal, glass rod, tweezers, a horse chestnut twig, waste container Chemicals: Distilled water Time: 15 minutes
Procedure (hidden for students) a. Put a small horse chestnut twig into a snap capped vial. b. On one side of the twig, remove up to half of the bark using the scissors.
c. With the twig and loose bark in the vial add distilled water and stir several times with the glass rod.
d. Use tweezers to extract the bark pieces and twig from the snap capped vial so that it only contains water with the extract of horse chestnut.
e. Place the snap capped vial containing the horse chestnut extract and a second snap capped vial containing only distilled water on a cyanotype filter paper and expose it to sunlight.
Disposal: Collect the blue filter paper in a waste container.
Keep the horse chestnut extract for subsequent experiments. Explanation: In addition to sunscreen through reflec-
tion, some chemical substances provide light protection by absorbing sunlight and, as a result, reduce the intensity of UV radiation. UV-active substances such as 4-aminobenzoic acid derivatives are used in sun cream for this purpose. Selected substances contained in plants also absorb UV radiation and show the phenomenon of fluorescence. These fluorescent dyes are characterized by an extended conjugated π-electron sys- tem, which does not absorb visible light like other dyes, but is stimulated by short-wave UV radiation.
The horse chestnut contains a water-soluble, UV-active substance between its bark and wood. This chemical substance provides light protection by absorbing UV radiation. The left snap-capped vial contains water as a blank; the right snap-capped vial contains horse chestnut extract. The upper shade in the right picture is more yellow-green due to the sunscreen through absorption of the horse chestnut extract.
Page 42 GREEN TEACHER 109
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