Which part of the optical system is considered the first refracting medium of the eye?

The first refracting medium of the eye is the cornea. This transparent outer layer of the eye is responsible for the majority of the eye’s total optical power. When light enters the eye, it first passes through the cornea, which bends (refracts) the light to help focus it onto the retina at the back of the eye.

The precorneal tear film, while it plays a crucial role in maintaining the eye's surface and providing lubrication, does not constitute the primary refracting medium. It assists in creating a smooth surface for the cornea, but the actual refraction is predominantly done by the cornea itself.

The lens further refines the light that has already been bent by the cornea, adjusting focus for near or distant objects, but it is not the first medium that light encounters. The iris regulates the amount of light that enters the eye but does not refract light in the same way as the cornea does. Thus, the cornea is key in the eye's optical system, making it the correct answer, as it is the first structure involved in bending incoming light.