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4:05
Transcript
0:06
Have you ever noticed something swimming
in your field of vision?
0:10
It may look like a tiny worm
or a transparent blob,
0:14
and whenever you try to get
a closer look, it disappears,
0:18
only to reappear
as soon as you shift your glance.
0:21
But don't go rinsing out your eyes!
0:23
What you are seeing is a common phenomenon
0:26
known as a floater.
0:28
The scientific name for these objects
is Muscae volitantes,
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0:32
Latin for "flying flies,"
0:34
and true to their name,
they can be somewhat annoying.
0:37
But they're not actually bugs
or any kind of external objects at all.
0:42
Rather, they exist inside your eyeball.
0:45
Floaters may seem to be alive,
since they move and change shape,
0:48
but they are not alive.
0:50
Floaters are tiny objects
that cast shadows on the retina,
0:54
the light-sensitive tissue
at the back of your eye.
0:58
They might be bits of tissue,
0:59
red blood cells,
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1:00
or clumps of protein.
1:02
And because they're suspended
within the vitreous humor,
1:06
the gel-like liquid
that fills the inside of your eye,
1:09
floaters drift along
with your eye movements,
1:11
and seem to bounce a little
when your eye stops.
1:15
Floaters may be only
barely distinguishable most of the time.
1:19
They become more visible
the closer they are to the retina,
1:22
just as holding your hand closer
to a table with an overhead light
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will result in a more
sharply defined shadow.
1:29
And floaters are particularly noticeable
1:31
when you are looking
at a uniform bright surface,
1:34
like a blank computer screen,
1:36
snow,
1:37
or a clear sky,
1:38
where the consistency of the background
makes them easier to distinguish.
1:43
The brighter the light is,
the more your pupil contracts.
1:46
This has an effect similar
to replacing a large diffuse light fixture
1:50
with a single overhead light bulb,
1:53
which also makes
the shadow appear clearer.
1:56
There is another visual phenomenon
that looks similar to floaters
1:59
but is in fact unrelated.
2:02
If you've seen tiny dots of light
darting about
2:04
when looking at a bright blue sky,
2:07
you've experienced what is known
as the blue field entoptic phenomenon.
2:12
In some ways,
this is the opposite of seeing floaters.
2:15
Here, you are not seeing shadows
2:17
but little moving windows
letting light through to your retina.
2:21
The windows are actually caused
by white blood cells
2:24
moving through the capillaries
along your retina's surface.
2:28
These leukocytes can be so large
that they nearly fill a capillary
2:32
causing a plasma space
to open up in front of them.
2:37
Because the space
and the white blood cells
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are both more transparent to blue light
2:41
than the red blood cells
normally present in capillaries,
2:44
we see a moving dot of light
wherever this happens,
2:48
following the paths of your capillaries
and moving in time with your pulse.
2:52
Under ideal viewing conditions,
2:54
you might even see what looks
like a dark tail following the dot.
2:58
This is the red blood cells
that have bunched up behind the leukocyte.
3:03
Some science museums have an exhibit
which consists of a screen of blue light,
3:07
allowing you to see these blue sky sprites
much more clearly than you normally would.
3:13
While everybody's eyes experience
these sort of effects,
3:16
the number and type vary greatly.
3:18
In the case of floaters,
3:20
they often go unnoticed,
as our brain learns to ignore them.
3:24
However, abnormally numerous
3:26
or large floaters
that interfere with vision
3:29
may be a sign of a more serious condition,
requiring immediate medical treatment.
3:34
But the majority of the time
entoptic phenomena,
3:37
such as floaters and blue sky sprites,
3:40
are just a gentle reminder
that what we think we see
3:43
depends just as much
on our biology and minds
3:46
as it does on the external world.
— end of transcript —
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