Floaters are visible either because of the shadows imperfections cast on the retina, or because of the refraction of light that passes through them, and can appear alone or together with several others as a clump in one's visual field. They may appear as spots, threads, or fragments of "cobwebs", which float slowly before the observer's eyes, and move especially in the direction the eyes move. As these objects exist within the eye itself, they are not optical illusions but are entoptic phenomena (caused by the eye itself). They are not to be confused with visual snow, which is similar to the static on a television screen, although these two conditions may co-exist as part of a number of "visual disturbances" which include starbursts, trails, and afterimages.