Question

Please answer the following question:
Window glass is colorless in the usual thickness used, but appears slightly green when viewed from the edge due to small amounts of Fe³⁺ . Why is this so?

Answer 1

The green color you see is mostly due to impurity (Fe³⁺). There's an additional factor which enhances the effect. Ordinary glass has an index of refraction high enough (about 1.5) that for a squared-off piece of glass (with all corners at right angles) any light coming in at an edge will be totally internally reflected until it reaches another edge (or gets scattered or absorbed along the way).

So looking at the edge from any angle, you see light that's bounced off the flat surfaces of the glass many times, making its path length in the glass much longer than it would otherwise be. This wouldn't happen with a substance having a smaller index of refraction. If you put the glass in water , for example, and look at the edge at a grazing angle, you will be able to see out through the flat surface of the glass (and the edge won't appear green). As you look at less grazing angles, total internal reflection will begin and the glass will start to look green again.