For an in-depth explanation of why we can see through glass, check here on ScienceLine. The portion relevant to the current question is that we can see through glass because glass does not absorb light, because light cannot excite electrons in glass, and glass only reflects light that hits it from particular angles. As described in another answer on ScienceLine, there are two kinds of reflection, specular and diffuse. The first happens for light hitting large flat regions so the light bounces off at similar angles (essentially how mirrors work, and also producing glare from windows/screens/water). The second occurs when light is reflected in many different directions.

Glass typically comes in large panes or sheets which are very flat and also with very little other "stuff" in them. This means there is nothing to diffract the light at odd angles, and it simply passes through (transmits). Sand, having the same chemical composition as glass, also does not absorb light. However, sand does not come in large sheets. Instead it comprises small, irregularly-shaped grains that have sides in many orientations. Additionally, sand grains can have cracks or pieces of other materials embedded in them; these can reflect light in yet other directions. Therefore, light which reflects off of the sides of sand grains does not travel in a single direction, and that reflected light reaches our eyes at different times and from different directions, so no sensible image is produced. Some of the other materials within sand might absorb light too, further reducing the light which would pass through sand.

You can see through sand, too, if it's thin enough.

Sand is made of tiny pieces of quartz (crystallized glass). Light that strikes the sides of pieces of quartz (or glass) bounces off of it. You know this from looking at a window: you can use a window as a mirror if you look at it from the right angle. In sand on a beach, there are so many pieces that your eyes can't make out the image of the light that bounces off of the sides, so it looks white or yellow, the color of the light that is hitting it (usually sunlight). If you were to look at a grain of sand under a microscope, it would look like the little piece of quartz that it is.

To make window glass, sand is heated to ~1790 degrees C, which turns it into a liquid. But, the sand that is heated is VERY pure - almost 100% quartz.

You can see through glass because it is amorphous - meaning that it doesn't have a crystalline structure. The crystals of quartz that make up sand, however, do have a crystalline structure. So, light travels differently through something that has the same composition (all quartz is SiO₂), but different structures (glass isn't crystalline, but quartz makes crystals).

Funnily enough, there IS a way to actually see through sand grains - you just have to slice them thin enough! In order to get a REALLY good look at a rock, geologists slice a rock SUPER thin (.03 mm), put it on a slide, and look at in under a microscope. See here. We call this a 'thin section'. Below, is a picture of a sandstone in thin section (the image is 5 mm across). You can see that the grains of sand (quartz) are actually colorless!

So, if the quartz that makes up the sand is colorless, then the glass made out of the sand would be colorless as well. And, since glass isn't crystalline, we can see through it!

Thanks for your question, and I hope you're having a good summer!
Cheers,

Hi Linden, great question! Glass is actually made of sand. They heat it up really, really hot (3090°F) until it melts. When it cools back down, it changes from the yellow color it used to be to clear. Then, while it’s still warm, they shape it into a window/vase/table or whatever it needs to be! That’s why we can see through it.