system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is our vision blurred when we open our eyes under water?
gpt
It has to do with the refraction index between water and air. When light hits a medium that it can pass through, the direction is slightly altered. It's like when you stick your hand in a fish tank and it looks like it has shifted to the side. 

To put it simply, our eyes have evolved to compensate for the refraction index in air, and our brains are used to it. In water, the refraction index is higher (the light "shifts" more) so our brain isn't able to make sense of what we see. The reason goggles work is the layer of air between your eyes and the water. The air refracts in a way that our brains are used to. The light in the water is still being shifted, but it is all relative and we don't have anything to compare it to.