Refraction is the bending of light due to a changing medium density and speed through which the light travels through, as mentioned in more detail on the page Refraction of light. The eye has many parts within it that allow for refraction to occur including the cornea, aqueous humour, lens and vitreous humour. The structure and function of these parts of the eye can be found at Anatomy of the Human eye.
When entering the eye, light first travels through the cornea. As the density of the cornea is close to that of water the light is slightly refracted as it travels through the cornea. From the cornea the light travels through the Aqueous humour and is further reflected due to the density of the fluid like substance which makes up the Aqueous humour. Once through the Aqueous humour the light move through the lens. The lens, being a transparent biconcave disc with a high density, is able to refract to light a specific amount to allow the focusing of the light to occur on a specific position in the retina called the fovea. It does this by adjusting its shape and thickness causing either more or less refraction to occur. On its path to the retina the light travels through the Vitreous humour where, similar to that of the Aqueous humour, the light is slightly refracted.
From this process it is evident to identify how the
cornea, aqueous humour, lens and vitreous humour all work together in order to refract the light enough to allow the focusing of the light to occur on a specific position in the retina called the fovea.




http://www.youtube.com/watch?v=15P8q35vNHw&feature=related is the video above representing the process light experiences as it travels through the eye. It explains the process mention above and supports it with visual aid.

Aubusson, Eileen Kennedy Peter. Biology in Context: the Spectrum of Life. Victoria : Oxford Uni. Press., 2001. Print.

"Lesson Ideas: Optics: Worksheet 2." Worksheet 2: The Human Eye - structure and function.. N.p., n.d. Web. 22 June 2010. <http://www.internal.schools.net.au/edu/lesson_ideas/optics/optics_wksht2_p1.html>.