Don't believe your eyes

24

Fig. 2.

mi whizz particles are disrupted: more particles fall than rise, and the depression is gradually filled with snow.

Similar processes occur when snow, carried by the wind, encounters some obstacle in its path, such as a tree. In front of the tree trunk, on the side from which the wind is blowing, an upward movement of air occurs. It leads to the formation of a deep depression on the windward side of the trunk and on the sides of the snow surface. In front of this notch and slightly behind the trunk, where the wind speed is lower, on the contrary, an elevation is formed.

The described phenomenon is used to protect sections of roads passing through hollows from snow. At a certain distance in front of the excavation or hollow on the side from which the wind blows, install a fence made of rickety sheets. Behind the shields, a calm zone with a uniform weak wind is formed, in which all the snow carried by the wind is completely deposited.

The movement of sand dunes is also obligated. A strong enough wind blowing onto a sand dune lifts the sand from the windward side. On the leeward side, where the wind speed is lower, the sand falls down. As a result, over time, the dunes gradually move in the direction of the wind - “coaching”.

DON’T BELIEVE

TO YOUR EYES

On the pages of our magazine («Kvant» No. 10, 1970) we have already written about critical illusions.

Here is another interesting example of such an exchange of vision. Concentric circles are drawn on the cover of the magazine. However, due to the background consisting of colored spirals, the lines of circles, sometimes transformed, are perceived by the eye as spirals.

Using a compass, you can easily verify that this is a real image of a circle.

It is curious that if you illuminate figure with short flashes of light, then after a certain flash duration the illusion disappears.

The same illusion will arise when looking at figure placed below. The circles shown here also look like spirals.