Index of refraction
Different materials have different indicies of refraction. Air has an index of refraction of ~1, while water has an index of refraction of ~1.33, the lens of your eye has an index of refraction a bit larger than water.
When light travels from a medium with one index of refraction into a medium with a different index the path of the light is bent- with the amount of 'bending' given by Snell's Law.
http://en.wikipedia.org/wiki/Refraction
When you change the index of refraction of the medium your eye is immersed in you are changing the optical properties of the eye/medium system. You are changing the strength of the lens.