I can see the light! Wait, no. Wait, yes. Wait, what?!
January 19, 2010 10:09 AM   Subscribe

Why are my eyes messing with me?! Why do I see something that's not there? Or why don't I see something that is there? Specific details inside.

Last night I used a bit of electrical tape to cover up a blinking LED on my computer monitor. After turning the overhead light off to check that the blinking LED was covered enough, I noticed something crazy.

When I look directly at the LED, I cannot see any light emitting from the LED whatsoever -- just as if it was unplugged. However, if I look away and focus on something else in the room, I can see the LED blinking out of the corner of my eye very prominently (almost as if there was no tape covering it at all). Quickly glancing back at the LED, I notice that I cannot see any light whatsoever.

So the curious me decided to find the reason. Reflection? Angle? Blind spot? Here's what I've tried:

1) I moved around my entire room, testing it in different spots, standing, sitting, leaning, everything. It does the same from every place in my room.

2) I checked for reflections. No reflections whatsoever (neither from an external source or the LED itself).

3) I tried all different angles and it acted the same every time. The piece of tape is small and does allow for some light to escape around the edges. However, if it was due to the angle, I should be able to see the light with my head parallel and directly inline with the edge of the LED (the most likely place any light could escape), but I still did not see any light whatsoever.

4) I tried different positioning. I turned my head to look away and back again. I then kept my head still and moved my eyes to look away and back again. It acted the same.

5) I tried focusing on only one inch away from the LED and it was the same as focusing ten feet from the LED. The only time I cannot see the LED blinking is when I focus directly on it.

6) I thought I may have found my blind spot by fluke. But the fact I can see it at any depth, any position of my eyes, any position of my head suggests not based on my understanding of a blind spot.

7) I brought someone else into the room and they had the exact same experience I had with all my tests.

So why is this?! I know it's stupid, but it's driving me crazy. I'm not looking for a solution to cover up the LED (I'll just use a bit more tape), I want to know why I can see it, but not when looking directly at it. There's got to be some scientific reasoning. Could it be something similar to a blind spot? Is it a trick my eyes are playing on me? What's going on here? Any and ALL ideas are very welcome. :)

A couple clarifications:
1) The light is definitely blinking under the tape at all times at a consistent rate (about one blink a second).
2) The piece of tape is very small and does allow for some light to escape around the edges (as explained above). But it comes does to this: if I focus directly at the LED, I don't see any light at all; if I focus away from the LED (anything greater than a quarter inch from the postiion of the LED, I can distinctly see the flashing light.
3) This occurs every single position in my room within a line of sight with the LED (although the intensity of the light may change slightly at each position).
4) The exact same scenario occurs for two people. I'll be able to try it with a third when I get home.

Any ideas at all?
posted by Kippersoft to Science & Nature (9 answers total) 4 users marked this as a favorite
Best answer: You have more light sensitive cells in your periphery then in the center of your eye.
posted by Dmenet at 10:12 AM on January 19, 2010

Best answer: Ditto dmenet. Simply put, you can see dimmer objects out of your peripheral vision than you could by looking directly. Stargazers take advantage of that fact by using a technique known as averted vision.
posted by deadmessenger at 10:18 AM on January 19, 2010

Best answer: Cones, which give you color vision, are in the center of your retina; rods, which give you black and white vision but are more sensitive to light, are around the outside edges.
posted by weapons-grade pandemonium at 10:23 AM on January 19, 2010

Dmenet is right. If you've ever been in a rural area at night and looked at the stars, you've probably noticed the same effect. They're much less bright when you look directly at them than when you see them from the corner of your eye. I suspect nothing is wrong with your eyes—especially because it works for two people.
posted by k. at 10:25 AM on January 19, 2010

Here's a fun trick: Have someone sit in a chair looking straight ahead. From behind them, move a brightly colored object gradually around into their field of vision. Ask them to say "Now" when they see it. Then ask them what color it is.
posted by weapons-grade pandemonium at 10:26 AM on January 19, 2010 [1 favorite]

Best answer: Yep -- you can do the star thing at night yourself -- when stars are first starting to come out, go outside and look up, you'll notice stars out of the corner of your eye but when you focus on them, you won't be able to see them.

The cells that do vision are called photoreceptors. There are two types: rods and cones, and they are distributed differently, and have different jobs.
-Cones are mostly focused in the middle of your vision. These do color perception and have a much higher acuity for detail viewing.
-Rods are absent in the exact center and come up around the periphery of your vision. These are much more sensitive to light, but have poorer acuity, and can't see color.

Here's a graph of the distribution -- the fovea is the center of your vision, where it's in perfect focus. The gap a little to the right of the fovea is the blind spot, where all the nerves leave your eye so there's no room for any photoreceptors (which is an entirely different story).

Basically, this is very normal and expected for a dim light.
posted by brainmouse at 10:31 AM on January 19, 2010

Response by poster: You guys (and or girls ;) are awesome! I figured it was something like this, but hadn't heard of it before and had no idea how to search for it.

Now that I think of it, I have noticed this before while star gazing a bit. I never really thought it about until now though. Perfect chance to learn something new. Thanks again!
posted by Kippersoft at 10:38 AM on January 19, 2010

You have more light sensitive cells in your periphery then in the center of your eye.

Did you mean motion sensitive, rather than light sensitive? Because I'm pretty sure its motion sensitive...hence the "blinking".
posted by hal_c_on at 1:11 PM on January 19, 2010

I think the blinking of the light could have played an important role, too.

A long time ago, my mate and I rescued a little, totally black neighborhood kitten which the family right next door to us ended up adopting, and for years, until they finally moved, whenever I came home on my bicycle and it was out, that cat would run and gambol on the grass of its yard, then my yard next to my front wheel as I rode past its house to my sidewalk, where I would put the bike down and pet it on my steps for a while before I went into the house.

One day I came home after dark on an overcast day when the streetlight in front of my house was burned out and very few lights in anyone's house happened to be on. I had no light on my bike and the only light I could see around me was some glimmering on the crystal planes in the sidewalk from car headlights more than a block away.

But even though their yard and my yard were totally dark to me, I could somehow still see the motion of the cat running along beside me despite the fact I couldn't see any shapes whatever and had no perception of light at all from that direction (and the abstracted motion was eerily beautiful, by the way, as if a chunk of pure darkness had come alive in the likeness of a cat).

When I got to my walkway and paused for a moment, a car drove by, and there was the cat sitting on the step looking up at me like 'why aren't you down here petting me the way you always do?'.

Rod cells
are also the primary motion detectors of the eye. I was far from being fully dark adapted at the time of this incident, so I suppose I can't really assert from my own experience that this is the case, but I bet that motion detection is actually more sensitive than light perception out there on the edge.
posted by jamjam at 1:11 PM on January 19, 2010

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