As you enter an old barn, you see a shaft of bright light shining down from the space between two of the boards that are part of the barn’s wall. A myriad of specks of light dance about within that shaft of light. Some of the specks are very dim, while others shine brightly. These points of light are specks of dust floating in the air, and the largest specks of dust are the brightest points of light. In fact, the shaft of light is only visible because millions of specs of dust collectively reflect the sunlight, like drops of water in a fog