Evolution of Earth
Scientific Explanations...
Work, Energy, & Environment
Global Warming vs. Ozone Depletion


Give scientific explanations for the following facts. Express the concepts, facts, and chain of logic clearly. Draw supporting figures if useful.

  • People in the tropical areas have higher chance of developing cataracts in their eyes than people in the temperate zones

Explanation: A cataract is the eye lens turning opaque, leading to loss of sight. This happens because of changes in the proteins in the lens. Visible light at most ordinary levels can not cause these changes, but ultraviolet light can. Solar radiation contains ultraviolet. Much of this is filtered by the ozone layer in the stratosphere. However, some ultraviolet does get through. The tropics receive more direct sunlight, and therefore a greater intensity of solar radiation. So, the amount of ultraviolet filtering through the ozone layer is higher in the tropics. Thus, people in the tropical zone have more ultraviolet falling on their eyes, and therefore have a higher chance of developing cataracts.


  1. Most leaves appear green to our eyes.

  2. Chlorofluorocarbons give rise to "holes" in the ozone layer.

  3. There is smog in Los Angeles.

  4. People in cities with higher traffic density have a greater chance of developing respiratory problems than people who live in well-ventilated houses in pristine areas.

  5. Energy and matter are conserved (as far as we know), yet we are concerned about "running out of energy".

  6. Deforestation contributes to global warming.

More challenging:

  • In fair weather, the sky is blue.
  • Sunsets are redder after a volcanic eruption.
  ©Copyright 2003 Carnegie Mellon University
This material is based upon work supported by the National Science Foundation under Grant Number 9653194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.