Question

Why we cannot see air
And fish cannot see water

Answer 1

We cannot see air and fish cannot see water because both air and water are transparent and also humans have refractive index as same as the air and fish similarly

Answer 2

Nope. Neither fish nor humans see water orair, because both water and air, are transparent.

Both fish and humans can see the interfacebetween water and air, because when light travels through the interface of two transparent mediums, with different indices of refraction, the light bends (changes direction), based on how much it slows downor speeds up (depending on whether it passes from a medium with lower to a medium with higher index of refraction, or vice versa).

At the interface between two transparent materials (like water and air), there is a critical angle, which occurs when the angle that the light its the interface at is so large that instead of passing out of the material, it gets reflected and stays inside it. Beyond this angle you get total internal reflection, and it’s why when you look up towards the surface of the water from below, it’s reflective unless its straight up and flat.