Why is glass transparent? Or even some plastics?

Why is Glass Transparent?

This progression from ordered to random organization is the primary reason that light can pass through liquids and gases. Just like bricks stacked neatly on top of one another, the ordered molecules of most solids are virtually impenetrable to light waves. Depending on the substance, the light waves will be reflected, scattered, absorbed or, more likely, some combination of the three. But as the substance changes to liquid or gas and the molecules are not stacked neatly anymore, gaps and holes occur that allow portions of the light waves to pass through. The greater the randomness of the molecular organization of the substance, the easier it is for the light to pass through.

Glass is generally a manmade substance. Here is the basic way to make glass:
1. Take the most common glass material, silica, which is just plain old sand like you would find on the beach.
2. Heat it to an extreme temperature until it becomes liquid, then cool it.

The resulting substance has a molecular structure that is very random like a liquid yet that retains the strong bond and rigidity of a solid.

To find out more: http://science.howstuffworks.com/question404.htm


How is Glass Colored?
Glass is colored by

(1) impurities in the batch ingredients, or
(2) by one of three processes:

a. using a dissolved metallic oxide to impart a color throughout
b. forming a dispersion of some substance in a colloidal state, and
c. suspending particles of pigments to form opaque colors.

http://www.cmog.org/index.asp?pageId=759


Picture Viewing Through a Red Filter
http://www.pa.uky.edu/sciworks/physicspettingzoo/Red%20View%20Blue%20View.htm

It just struck me in an out of the blue fashion - we see the green filter because while green light is allow to pass through, it is also reflected off the white surface and transmitted into our eye through the same filter. Imagine having a black surface behind a filter, you would not see the color of the filter very well because when the green light hits the black surface, it will most likely absorbed.
Soundz sensible? =)