Why does water always seem shallower than it is?

Light is one of the most complex things in the universe. It is so complex that it even warps the fabric of space/time due to its speed. Light is how we see. Light heats our planet and makes our ecosystem work. Without it, life could not exist.

Light can also be deceptive. Light has the ability to go through semi-permeable objects. Fluids especially are susceptible to this. An interesting phenomena occurs when light goes flows through water, the most abundant liquid on the planet. When looking at an object fully submerged underwater, the object serving as a focal point, the water seems to be fairly shallow. But when you try to pick the object up, or enter the water, it is much farther down than your eyes tell you the object is. Why is this? The phenomena is a something called refraction.

Our eyes depends on light entering our pupils, which stimulates the optic nerve. The optic nerve transmits the signals it receives from light to our brain, which then configures these signals into a clear image of what we perceive as the world around us. We are wholly dependent on light to see and if that light is messed with or modified in some way, our most used, strongest sense might betray us.

What is refraction? Refraction is when light hits the water going one direction and is bent by the water to go a slightly different direction. A more complicated answer is that the light is bent by the water towards something called the normal line. The normal line is a line drawn perpendicularly from the surface through which the light is passing. In this case, that is up and down. When light passes from air to water, the light is shifted so it is going in a slightly more downward direction. 

An example of this would be a fish swimming underwater. We see the fish, we perceive it to be in a straight line in front of us underwater. But what is actually occurring is that the fish is farther down than we perceive it to be. The light getting refracted by the water bends down toward the normal line, picking up the fish. The light then bounces back to our eyes, still carrying the refracted image of the fish. So what we perceive to be a fish out in straight line in front of us is really a fish at a much lower angled line in the water, causing an optical illusion to form. Our eyes never take refraction into account, only seeing things in terms of straight lines. Refraction is why water nearly universally seems shallower than it actually is at a glance. Here is an illustration to help explain refraction more clearly:

Other interesting things about light:

Light has many other fascinating properties as well. Did you know that as you move faster through space, time actually slows down? light, scientifically, is the fastest thing in existence. For it, time is nearly meaningless. If you were to travel at even 60% the speed of light, time would pass almost twice as slow for you, meaning twice as fast for everyone apart from you. So, if you travelled for an hour at roughly 60% the speed of light, two hours would pass for everyone else! The effect only strengthens as you approach the speed of light.

Light also warps mass and length for physical objects. The closer you travel to the speed of light, the heavier, relatively speaking, your mass becomes. The length of say, a pole you are carrying would also shrink, becoming shorter. 

Light has many fascination properties to it. Entire books have been written on the subject. Scientists still are not fully aware of all of light’s capabilities.