WEBVTT 00:00:06.830 --> 00:00:10.424 Have you ever noticed something swimming in your field of vision? 00:00:10.448 --> 00:00:14.061 It may look like a tiny worm or a transparent blob, 00:00:14.085 --> 00:00:18.052 and whenever you try to get a closer look, it disappears, 00:00:18.076 --> 00:00:20.763 only to reappear as soon as you shift your glance. 00:00:21.727 --> 00:00:23.792 But don't go rinsing out your eyes! 00:00:23.816 --> 00:00:26.393 What you are seeing is a common phenomenon 00:00:26.417 --> 00:00:27.449 known as a floater. 00:00:28.451 --> 00:00:32.593 The scientific name for these objects is Muscae volitantes, 00:00:32.616 --> 00:00:34.755 Latin for "flying flies," 00:00:34.779 --> 00:00:37.909 and true to their name, they can be somewhat annoying. 00:00:37.933 --> 00:00:42.045 But they're not actually bugs or any kind of external objects at all. 00:00:42.069 --> 00:00:44.331 Rather, they exist inside your eyeball. 00:00:45.209 --> 00:00:48.742 Floaters may seem to be alive, since they move and change shape, 00:00:48.766 --> 00:00:50.013 but they are not alive. 00:00:50.832 --> 00:00:54.649 Floaters are tiny objects that cast shadows on the retina, 00:00:54.673 --> 00:00:57.228 the light-sensitive tissue at the back of your eye. 00:00:58.077 --> 00:00:59.590 They might be bits of tissue, 00:00:59.614 --> 00:01:00.722 red blood cells, 00:01:00.746 --> 00:01:02.681 or clumps of protein. 00:01:02.705 --> 00:01:06.206 And because they're suspended within the vitreous humor, 00:01:06.230 --> 00:01:09.277 the gel-like liquid that fills the inside of your eye, 00:01:09.301 --> 00:01:11.580 floaters drift along with your eye movements, 00:01:11.604 --> 00:01:14.331 and seem to bounce a little when your eye stops. 00:01:15.291 --> 00:01:18.532 Floaters may be only barely distinguishable most of the time. 00:01:19.460 --> 00:01:22.711 They become more visible the closer they are to the retina, 00:01:22.735 --> 00:01:26.217 just as holding your hand closer to a table with an overhead light 00:01:26.241 --> 00:01:28.727 will result in a more sharply defined shadow. 00:01:29.555 --> 00:01:31.580 And floaters are particularly noticeable 00:01:31.604 --> 00:01:34.582 when you are looking at a uniform bright surface, 00:01:34.606 --> 00:01:36.219 like a blank computer screen, 00:01:36.243 --> 00:01:37.446 snow, 00:01:37.470 --> 00:01:38.814 or a clear sky, 00:01:38.838 --> 00:01:43.083 where the consistency of the background makes them easier to distinguish. 00:01:43.107 --> 00:01:46.761 The brighter the light is, the more your pupil contracts. 00:01:46.785 --> 00:01:50.698 This has an effect similar to replacing a large diffuse light fixture 00:01:50.722 --> 00:01:53.018 with a single overhead light bulb, 00:01:53.042 --> 00:01:55.323 which also makes the shadow appear clearer. 00:01:56.189 --> 00:01:59.744 There is another visual phenomenon that looks similar to floaters 00:01:59.768 --> 00:02:01.319 but is in fact unrelated. 00:02:02.382 --> 00:02:04.760 If you've seen tiny dots of light darting about 00:02:04.784 --> 00:02:06.977 when looking at a bright blue sky, 00:02:07.001 --> 00:02:11.098 you've experienced what is known as the blue field entoptic phenomenon. 00:02:12.273 --> 00:02:15.140 In some ways, this is the opposite of seeing floaters. 00:02:15.947 --> 00:02:17.578 Here, you are not seeing shadows 00:02:17.602 --> 00:02:21.028 but little moving windows letting light through to your retina. 00:02:21.982 --> 00:02:24.796 The windows are actually caused by white blood cells 00:02:24.820 --> 00:02:27.886 moving through the capillaries along your retina's surface. 00:02:28.696 --> 00:02:32.753 These leukocytes can be so large that they nearly fill a capillary 00:02:32.777 --> 00:02:35.907 causing a plasma space to open up in front of them. 00:02:37.132 --> 00:02:39.142 Because the space and the white blood cells 00:02:39.166 --> 00:02:41.696 are both more transparent to blue light 00:02:41.720 --> 00:02:44.961 than the red blood cells normally present in capillaries, 00:02:44.985 --> 00:02:48.041 we see a moving dot of light wherever this happens, 00:02:48.065 --> 00:02:51.924 following the paths of your capillaries and moving in time with your pulse. 00:02:52.829 --> 00:02:54.786 Under ideal viewing conditions, 00:02:54.810 --> 00:02:58.912 you might even see what looks like a dark tail following the dot. 00:02:58.936 --> 00:03:02.578 This is the red blood cells that have bunched up behind the leukocyte. 00:03:03.428 --> 00:03:07.679 Some science museums have an exhibit which consists of a screen of blue light, 00:03:07.703 --> 00:03:12.242 allowing you to see these blue sky sprites much more clearly than you normally would. 00:03:13.195 --> 00:03:16.195 While everybody's eyes experience these sort of effects, 00:03:16.219 --> 00:03:18.799 the number and type vary greatly. 00:03:18.823 --> 00:03:20.315 In the case of floaters, 00:03:20.339 --> 00:03:23.487 they often go unnoticed, as our brain learns to ignore them. 00:03:24.311 --> 00:03:26.335 However, abnormally numerous 00:03:26.359 --> 00:03:29.045 or large floaters that interfere with vision 00:03:29.069 --> 00:03:33.664 may be a sign of a more serious condition, requiring immediate medical treatment. 00:03:34.569 --> 00:03:37.597 But the majority of the time entoptic phenomena, 00:03:37.621 --> 00:03:40.210 such as floaters and blue sky sprites, 00:03:40.234 --> 00:03:43.623 are just a gentle reminder that what we think we see 00:03:43.647 --> 00:03:46.766 depends just as much on our biology and minds 00:03:46.790 --> 00:03:49.044 as it does on the external world.