system
Given the following post, respond as if you're commenting on Reddit:
human
why is it we can see through water clearly from the outside but can't see well while opening our eyes underwater?
gpt
The angle at which light bends at an interface depends on the refractive indices of both materials. This is Snell's law: sin θ_1 / sin θ_2 = n_2 / n_1. Since your eyes are designed to function in air, with a refractive index of about 1, when you immerse them in water, with a higher refractive index, light entering your eye isn't bent as much, which means it isn't focused properly.

This is why you can see just fine when you put goggles on - they mean your eyes are in air, not water, so light is focused as normal. (The interface between air and water in goggles is (usually) flat, so it doesn't affect the focus of the light.)