There are two optically transparent tissues in the human body: the cornea and the lens. But how they achieve this transparency is different, and both in turn appear to achieve this differently than in other optically clear materials like glass. The transmission of light through glass or other clear solid material is thought to occur because the differences in the energy levels of the molecular orbitals in the material, such as silicone dioxide in glass, are too high for light in the visible range of the electromagnetic range to be absorbed. So the visible-wavelength photons pass through unabsorbed. But from what I am reading, the transmission of light through the biological cornea and lens of the eye appear to have very different mechanisms. The cornea's mechanism seems to be that the collagen tissue making up the cornea has a perfectly ordered hexagonal cross-sectional arrangement, and the spacing between the collagen fibrils is always kept greater than wavelength lambda/2 to allow the phase of the waves to pass through. But the collagen in the cornea is no different than the collagen that's present in the sclera of the eye as well. The sclera is white and opaque because the fibrils are not regularly arranged as they are in the cornea. They are arranged haphazardly, and that makes all the difference. If the fibrils in the cornea are also disrupted in pathologic conditions, as in injury, trauma, or edema, the cornea will lose its transparency like the sclera. The lens, the other refractive tissue in the eye, seems to follow yet another mechanism. There is no collagen in the lens. It is made of proteins called crystallins. These are apparently clear proteins. But I have not been able to find the exact mechanism through which they maintain such optical clarity. Regular ordering of fibrils like with collagen are clearly not the mechanism, as these proteins are spread haphazardly in the lens and much more densely packed. But I read in one place that the lambda/2 criterion is also important for these proteins to maintain optical clarity, because when this is disrupted through fluid buildup in the lens or cross linking between these crystallin proteins, the lens loses its optical clarity and a cataract ensues. So here are my questions: 1) Does anyone know more about this lambda/2 criterion as another mechanism for maintaing optical clarity in materials, independently of the electron orbital energy levels? 2) Proteins are chemically/physically extremely complex structures. It shouldn't matter how they are arranged or spaced. You would think a huge amount of the photons should be absorbed by all the elelectrons in the amino acids of the protein, and the efficiency of light transmission through the biological cornea and lens should be very poor- and yet they are not. Why not?