# Why are the edges of a broken glass almost opaque?

Unfortunately I broke my specs today which I used in this question.

But I observed that the edges are completely different then the entire part of the lens. The middle portion of the lens was completely transparent but the edges appeared opaque (and I can't see through the edges). This image shows the same in case of a shattered glass.

The edges in the above picture are green and not transparent as other portions appear.

So why are the edges not transparent (in both the case of specs and the shattered glass)?

Edit : I would like to add that the edges of my specs were not green. They were just silvery opaque. I couldn't take a pic of it during asking the question but take a look at it now.

• Diffusion, eventually by reflection, of the light. Similar to soda drink foam or beer foam, just at microscopic scale. Or similar to sanded glass. Jan 14, 2021 at 10:36
• I did suspect your image to be some other glass'. How can some lens be broken into that many pieces and that big?
– lee
Jan 14, 2021 at 14:17
• Might I ask where you live? Because AFAIK eyeglass lenses have pretty much all been made of various shatterproof plastics for many years. Jan 15, 2021 at 17:55

## Because you're looking through more of glass

I'd like to just add to the other answers with some diagrams. We have an intuition that light beams travel in straight lines, so we tend to assume that the beam paths looking through glass might be as follows:

However, the actual paths of the beam due to refraction and total internal reflection look more like this:

Note that the beams that enter the face of the glass aren't significantly deflected, and exit the glass pretty quickly. However beams that enter the edge of the glass spend a lot more distance within the glass. As the beam spends more time within the glass, it has more of a path to be affected by impurities.

• Outstanding figures. This is how to answer a question through insightful visualization. Jan 15, 2021 at 23:01
• @Hannesh in case I use a biconvex lens should I get the same opaqueness in the middle than at the edges ? Jan 16, 2021 at 9:06

Have you ever noticed that at the aquarium, sometimes the thick glass tanks carrying fish appear green? Glass is made of silica and lime but sometimes it carries impurities, like iron oxide, that gives it a greenish appearance.

What’s happening in your example, is that when you look at thin glass like your glasses lenses (before they broke) they were clear because you were not looking through “thick enough” glass that shows this greenish hue.

But what is happening when the lenses are cracked and the edges are green and opaque? Actually, looking into the edges we notice refraction and more importantly, internal reflection.

This means that we are actually looking into the “thick part” of the glass through its edge parallel to its surface. Imagine picking up one of the broken pieces, and hold it as if you want to see through to the other broken edge.

The thickness is now the distance from one broke edge to the other, and since now you see more glass, you will notice the green colour.

• @Dr jh the edges of my specs were not green btw. The picture I added was from Google since I couldn't take the pic of my specs . Now I have added the pic of my specs. Jan 14, 2021 at 12:20
• @Ankit: Is it possible that you take several glass sheets and verify this answer? By looking onto say 10 glass sheets the thickness of the glass is ten-fold increased. Jan 14, 2021 at 16:46
• @Ankit, The green color of the edges of the broken pieces in your top picture is characteristic of soda-lime glass—about 90% of all glass manufactured. The green color is not usually thought of as desirable, but everybody tolerates it because (a) it's barely noticeable except at the edges of a sheet, and (b) soda-lime glass costs significantly less to make than other types of glass. The several different types of glass that are used to make most lenses, prisms, and other optical components don't have that color. Jan 15, 2021 at 1:45
• @Dr jh also does it have something to do with the curved shape of the lenses ? Will a straight glass show less opaqueness than a curved one ? Jan 15, 2021 at 4:23
• Iron oxide is a distinctive red-brown color. Why does it make glass look green? Jan 16, 2021 at 13:27

These can be the reasons:

• First of all, when you broke your specs, the edges became uneven so they(light rays) can't penetrate the glass properly. Due to which the light rays got reflected away by varying degrees and therefore it appears opaque. Note: But I don't think this is the reason why it appears opaque.
• Secondly, the green colour of your glasses can be from the impurities (like iron oxide) which make it appear green. Note: But again, I don't think that this is the possible reason because glasses that are used to make specs are highly purified ones unlike the ones used for normal uses like aquarium and all.
• Last and the most probable one is that the glass is undergoing total internal reflection. You know, the critical angle of glass is ≈ 42°. So, if the light rays enter with an incident angle larger than 42°, it will undergo total internal reflection. So it's literally(or bitterly ; ) trapped inside the glass due to which it might seem shiny and opaque (and may be green?). This is the same reason why cracks in a glass vessel or bottle looks shiny.

Edit:
So, as you've told that the broken pieces of your lens don't look green but silvery-white, the last point of mine will do it. To add to the point, it is not necessary that your lens has to break to see this silvery-white opaqueness. It will be visible even if the lens is looked from the sides.
And, maybe you can use this total internal reflection to find small bits of your lens scattered here and there. Just use a torch and shine it on the floor (or whatever the surface be) and if you see anything sparkling it will be glass bits. Hope this helps: )

• This is more complete. I would no say the first aren't the reason, it will depend on what exactly the observer see. Something like in the added photo (smooth and dark side) is certainly internal reflection, or more glass to see through, as also mentioned in another answer. Plus 1. Jan 15, 2021 at 9:44

They are not transparent because they are "rougher" and are reflecting/scattering more light as compared to a nicely polished surface which will transmitt more. In my research we experience the same when we're polishing nonlinear crystals. We first have a rougher polishing material to get the length/width down, but then the ends are rougher and not transparent. Then we need to use a finer polisher which polishes the surface without grinding down the size.

• by rougher do you mean that the edge of my broken lens has irregularities on it ? If yes then I would like to add that it has a smooth edge btw . The cut in the lens was smooth.. Jan 15, 2021 at 4:25
• I don't think that's the reason. It will look silvery-white even if the light comes from the other side. During reflection, incident ray, reflected ray, etc., lie on the same side of plane.
– lee
Jan 15, 2021 at 6:04