Myopia and hyperopia (short and long sight)
Two common defects of the eye are myopia and hyperopia. In order to understand these defects we first have to understand the eye.
The eye's conjunctiva, anterior chamber, lens and vitreous body all act on a ray of light, which gets refracted at each surface boundary.
Each material has a different refractive index, different principal focal lengths, different principal focal points and different centre lines.
(Please read Mammoth Memory Physics – Refraction section and Mammoth Memory Biology – the Eye to learn about the parts of the eye.)
At this level, we have to treat these different media in general terms as if they have one image position and one centre line of a lens (roughly in line with the cillary muscle) – but understand it's a lot trickier than this.
The below diagram is more like the ray diagram of a real eye but even this doesn't begin to show how amazing the eye is.