Inexperienced swimmers are often subjected to great danger only because they forget about one curious consequence of the law of refraction of light: they don't know what breaking like lifts all submerged objects above their true position. The bottom of a pond, stream, each of the reservoir appears to the eye upward almost one third of the depth; relying on this deceptive fineness, people often find themselves in a dangerous situation. It is especially important to know that children and people of small stature, for which the error in the determination of depth may be fatal.
The reason is that the refraction of light rays. The same optical law that gives half-submerged in water spoon islomania (Fig. 1), and causes the apparent upwelling. You can check it out.
Figure 1. The distorted image of a spoon, put in a glass of water.
Put friend at the table so that he could not see the bottom is standing before him a Cup. On the bottom put a coin, which, of course, will be hidden by the wall of the Cup from the eyes of your friend. Now ask your friend not to turn heads and pour in a Cup of water. Happens something unexpected: the coin will be for your guest visible! Remove water from a Cup with a syringe, and the bottom with a coin will fall again.
Figure 2. Experience with the coin in the Cup.
Fig. 3 explains how this happens. The bottom plot m seems to the observer (eye which is above water at point a) in the raised position: the rays are refracted and, passing from water into air, take in the eye, as shown in the figure, and the eye sees the plot on the continuation of these lines, i.e., m above. Than naklonnye go rays, the higher is m. That's why when viewed, for example, from the boat flat bottom of the pond we always think that it is the most deeply right below us, and all around - all smaller and smaller.
Figure 3. Why coin in the experiment Fig. 2 seems pripozdnivshijsja.
So, the bottom of the pond seems to be concave. On the contrary, if we could from the bottom of the pond to look at thrown over his bridge, he would seem to us convex (as shown in Fig. 4; the method of obtaining this photo will be discussed later). In this case, the rays pass from weakly refracting medium (air) in strongly refracting (water), and therefore the opposite is true than when the rays of the water in the air. For similar reasons, and the number of people standing, for example, near the aquarium, should seem to fish, not a straight line and an arc, facing his bulge to the fish. About how see fish, or rather, what they would see if they had the human eye, we will soon talk more.
Figure 4. In this form appears to be an underwater observer railway bridge over the river (with pictures professional wood).