Eye Floaters Optics

Question

Eye floaters are these annoying objects floating in someones eye, usually seen by someone experiencing them as squiggly lines and dots buzzing around, either dark or partially transparent (I experienced them recently).

They are clumps of refractive transparent collagen fiber debris that float in the vitreous body of the eye. The vitreous body is a ball of water that sits between the retina and the eye lens.

I have several questions regarding the optics of such objects:

1. How can they be seen as clear lines or dots if they are not focused by the eye lens given that they are positioned after the lens?
2. When given pupil dilation drops the floaters typically disappear while the pupil is dilated. Why is that the case? Since I would expect that with the pupil dilated the optical aperture is larger, hence resolution should improve and finer details should become sharper?
3. When squinting the eyes they become extremely sharp and visible in detail

Answer 1

They are visible because they cast shadows onto the retina, either by completely occluding the path of light or via refractive means. Imagine a dark box forming a camera obscura. If we place an object in the box, we will form a sort of image on the wall of the box by virtue of casting a shadow on it.

Your second and third questions are really the same. It is the same reason that decreasing the aperture of a lens will automatically increase the depth-of-field. A narrower aperture restricts the range of angles by which a ray can be transported from the object to the image plane. Consider that each point on the object/floater will be transported to a variety of locations on the retina, depending on the way that the particular ray enters the pupil. Narrowing the pupil decreases the size of this region on the retina, leading to a sharper image. See this poor diagram I drew:

$A$ represents the pupil, $B$ the floater, and $C$ the retina. You can see that the narrower pupil restricts the available paths mapping an incoming ray through a given point on the floater to the retina.

The inverse is the reason they become invisible upon dilation of the pupil - the pupil becomes sufficiently large that a given target location on the retina receives light from distant parts of the object, preventing formation of a useful image. Consider in our camera obscura analogy how narrowing the pinhole would bring the shadow-image into sharper view, whereas increasing the size of the pinhole would eventually render the shadow invisible.

Answer 2

Riley's answer is excellent; I'll add one piece that I think is relevant to your first question: the location of the floater within the eye matters. If the physical object is very close to the retina, it will cast a sharper, more focused shadow on the retina. If the object is close to the pupil, the shadow will be distributed across most of the retina and (I would guess) will probably be invisible. You can see this if you take a thin object like a piece of yarn outside and observe the shadow that it casts at different heights from the ground. Close to the ground, it's crisp and clear. Higher up, it blurs and disappears.
You are correct that the lens is not focusing the image of the floaters the way it does with images of the outside world. In fact, for the purposes of this question, we can ignore the lens completely and simply think of the pupil as a diffuse light source which illuminates the retina, which is why the comparison to shadows in sunlight works.