Refraction is the change in the direction of light due to the change in the medium’s refractive index traveled by the light. It is convenient to use water in a demonstration of refraction. But water alone is not the cause of refraction.
Flat-Earthers falsely claim that just because our atmosphere has water vapor in it, it will produce the same effect as any demonstration of refraction involving water. In reality, it requires far more reasoning than just that water is involved.
A classic demonstration of refraction involves a spoon in a glass of water. The spoon appears broken at the surface, and the submerged part appears larger. The demonstration involves water, but it is wrong to assume it occurs only because water is involved.
In reality, the water’s refractive index is higher than the surrounding air. The difference between the refractive indexes and the curved glass surface bends the light’s direction, makings the spoon appear larger.
Whenever there is an observation that should not happen if Earth is flat, flat-Earthers often attribute it to refraction, using the fact that our atmosphere contains water as a pretext. They would set up a demonstration involving lenses & mirrors and use it as “evidence” it can happen on a flat Earth. In reality, it is just a false analogy.
References
- Refraction – Wikipedia