Can we half-close our eyes such that they don't vibrate?
If not, then why we are unable to half-close our eyes properly?
Alternatively, why isn't it possible to keep the upper eyelid close to the lower eyelid without it vibrating?
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Sign up to join this communityThe motion of the eyelid is driven by the levator palpebrae superioris, i.e. elevating muscle of upper eyelid, and it can be positioned to the intermediate, half-closed positions. At least I can do it.
Yes, this muscle tends to shake or oscillate simply because it's a very weak muscle. It's similar with the muscles controlling the motion of the little finger (or ring finger). Vibration and "noise" is present in all muscles but the stronger muscle you have, the more accurately the random motion is averaged out.
If you connected $N$ eyelid muscles so that they move in unison, the noisy vibrations would be reduced by the factor of $\frac{1}{\sqrt{N}}$. This is the well-known scaling of the statistical errors in physics etc.
I would think that this question was totally appropriate at the Physics Stack Exchange.
The upper eyelid position is governed by two muscles (tarsus sup and levator palpebrae sup) working against one another, but they are at a 90 degree angle:
See Wikipedia on tarsus.
When you are closing your eyelid, it is the tarsus that contracts and reduces its length. If you try to maintain the upper lid in a half-closed position, you need to counteract with the levator muscle. But because of the nearly 90 degree angle, a little force of the levator needs to be counterbalanced with a strong levator force, simply because of trigonometry:
You see above (upside down compared to eyelid muscles) that for a constant force of the levator (F, down), the force of the tarsus to the left and right need to be doubled when you go from 30 to 20 degree angle of the "eyelid". These variations grow even larger when the angle goes flatter, in fact, it is a $1/\tan \alpha$ singularity [EDIT]as $\alpha$ goes to zero[/EDIT]. Tough maths problem, and also tough control problem for your nerves!
And definitely, this question is very much physics!