Example:

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.

• In fair weather, the sky is blue.
• Sunsets are redder after a volcanic eruption.