Actually, there is a fact that proves our eyes weren't "designed" at all, because they are far from being perfect. Their design contains many oddities or absurd details we inherited through evolution, but they weren't removed because they weren't impairing enough.
For instance, there is a layer of cells in front of the retina. Octopi eyes don't have this defect, they are better constructed than mammal ones.
Rods cells are in fact made of several disks stacked up together, each covered with these proteins (here rhodopsins). The more light, the deeper it gets into the cell, and thus the more disks are eventually affected. Hence, the rod cell transmits a signal that is conveyed to the brain as a single pixel whose grayness vary according to the amount of light.
Cones cells perceive colours. They are also made of several disks stacked up together, but of increasing diameters, hence the "cone" shape. The light here is diffracted like in a prism, since the goal is colour recognition. Again, the more disks touched by light, the more intense the primary colour will be (whether the cell is configured to recognize blue, red or green wavelengths: you have three variants ; and the cone angle and geometry will be slightly different each time, because the required diffraction won't be the same).
And after that, the brain processes all the pixels together: first, a black and white image (rod cells) that gives great acuity and precision, and just after that it "paints" it with the informations received by the blue, the red and the green layers (cone cells, which are less numerous and less efficient than rod cells)...
So you see, when you consider each phase separately, the mechanisms used in our eyes use simple chemical and optical properties, nothing really fanciful!