A question about optics...
Asked by
lilikoi (
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February 25th, 2010
In college, I noticed my eye sight beginning to deteriorate. I was once able to sit in the back of the lecture hall and see the board clearly, and then sat in the front of the same room and could barely make out a word on the board.
In the transition period between having 20/20 vision and absolutely needing glasses to read something on the board, I made an interesting discovery.
I found many lectures quite boring, and often did not get enough sleep the night before, and so I did a lot of yawning in class. What I noticed was that when I yawned, my eyes would naturally tear, and when the tears would well up in my eyes, my vision would be miraculously restored seemingly to 20/20 as long as I kept the water in my eyes.
I also know that when you put a fish in a tank and look at it from above, it looks much smaller than if you were to look at it from the side of the tank. Water obviously distorts vision (how?), and yet it amazed me that simple tears would distort vision perfectly enough to compensate for my failing eyesight.
How does this work?
And then, how does this work?
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11 Answers
The surface tension of the water pulls it into a lens-like shape, changing the focal point on your retina.
That is similar to how a contact lens works right, where the lens is shaped to change the focal point of the retina?
Okay, but how is it that the water forms a lens-like shape that is just what you need to be able to see clearly? Is there a large margin of error in that many different curves would cause me to feel like my vision is 20/20 (I can’t be sure exactly how good of a correction I got doing this, but it was pretty crisp)? Or will the surface tension of water always pull into the same shape within the confines of the shape of my eye so that if I were to do this now, I wouldn’t be able to see as clearly?
The focal length just happened to be correct. Try looking through a tiny hole. That changes the angle at which the light enters your pupil, also.
If you think that’s weird I’ve got an interesting story related to little holes (sort of).
My second year of college I started wearing glasses to class and realized the difference it made. One lecture I forgot them and sat in my usual spot near the back of the class. I couldn’t make out the board very well and I guess I was bored so I put my hands to my face. I noticed that while looking between the fingers of my hands I was able to make out writing on the board that I could not without my hand. After some experimentation I found that the most effective method was to put two fingers up between my eyes with the outer edges at the very edge of my inner vision, right next to my nose. I could move them outward or inward depending on the distance I was trying to see. I had a friend who bought pinhole glasses instead of prescription glasses around the same time and the idea clicked. They’re pretty cool and much cheaper.
Squinting tends to help sometimes though this may be more related to altering the shape of your eye due to pressure than the diffraction that happens around your eyelids.
Actually, the smaller the aperture, the greater the depth of field.
It’s been roughly 5 years since I took optics and I’ve used it so little that all I’ve retained are the very basic concepts anymore. My friend had mentioned that pinhole glasses are supposed to permanently alter your eyesight, I was skeptical (it seems with good reason, studies have shown no conclusive evidence of this).
You are correct. They just provide the narrow aperture that deepens the field.
@dpworkin is correct. I have bad astigmatism and am glasses dependent, but I can actually get pretty good vision by pulling the corner of my eyelid such that I push on the cornea. That changes the shape of the cornea and the path of light so it’s momentarily clearer.
I used to make a camera obscura in my house when I was a kid. When my parents would get something with a big box, I’d climb inside of it and poke a pinhole. I’d then position it so I could watch a projected image of the TV upside down and backwards. Yes, I was a geek even as a kid.
First of all, congrats for having such a scientific temperament.The observation was really nice.The tear film on your eyes are nothing, but acting as concave lenses or contact lenses.The water film diverge the light rays back to the retina.In your case, the image is formed in between the eye lens and the retina.This is what makes the vision blurred and that’s technically known as Myopia.
You could try one more thing and see the changes.Put a finger on the skin at the side of your eye where the eyebrow ends and pull it back gently.This would better the vision till you keep the skin pulled.In this case, your eyeballs are reshaped and contact, which again allows the image to be formed nearer to the retina , but don’t over do it.It’s just for the observation sake.
Visit an eye doc and get the glasses or lens as per your convenience.There are few Yoga for eyes which cures near vision.You could try those also, but don’t delay in visiting a doc.I made a delay after an accident at the age of 14 and my vision worsened to a very bad limit of -4D just within months after that.
Best wishes.
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