The goggles hold air against your eye.
Work Step by Step
Your eye has evolved to operate in air. That is, there is a certain amount of refraction that takes place when light travels from air into your eye. The light speed change is from "light speed in air" to "light speed in eye". However, if your eye were immersed in water, then the light speed change would be from "light speed in water" to "light speed in eye". Light would be refracted less than it is normally, and your vision would be blurry. Goggles hold an air pocket against your eye and ensure that the light entering your eye is coming from air.