When you look at a stick partially immersed in water, the stick appears bent. Light from that part of the stick immersed in water is refracted away from the normal as it passes from water to air on the way to your eyes.
The image of the bottom of the stick appears to be closer to the surface of the water than it actually is.
From Snell's law,
Since
is very nearly 1, we can write
From the diagram above, if
is small ,
Looking into a swimming pool 4 m deep, taking the refractive index of water as 1.3,