A little sun in a bucket of water. Drawing, description

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A little sun in a bucket of water. Children's Science Lab

Children's Science Lab

Using this telescope, Kharkov astrophysicist Viktor Petrovich Vasiliev takes photographs of the Sun.

Let's mentally set up such an experiment. Pour any sufficiently viscous liquid into a plate: for example, glycerin or oil. Imagine now that the plate began to rotate. If the speed of rotation is constant, then the surface of the liquid will take the exact form of a paraboloid of revolution. Why don't you have a mirror! And it does not need to be sharpened or polished. Such a device was first proposed by the famous experimental physicist Robert Wood.

But here's the problem: how to make a plate with liquid rotate strictly evenly, without shocks and vibrations that would nullify the correctness of the shape of the liquid mirror? A simple fixing of the container on the axis of the engine is indispensable here. Even Robert Wood himself failed to solve this problem to the end. But we still try!

A little sun in a bucket of water

Let's make a container out of tin like a bucket. We drill two holes at the very bottom of it and install two hoses in them, having previously sealed it, as shown in the figure. Connect both hoses to a small centrifugal pump. Pour water into the container - almost to the brim - and turn on the pump. As soon as a small whirlpool forms in the container, a plate filled with a viscous liquid is placed on the surface of the water. Caught up by the whirlpool, it begins to rotate evenly. Of course, in order to achieve a high-quality mirror, you will have to tinker: install adjustable clamps on the hoses, find the optimal operating mode for the pump, carefully select the size of the plate. And besides, in order to eliminate vibration, the pump itself must be placed on a shock-absorbing cushion - for example, rubber or foam rubber - away from the mirror.

But such a mirror cannot be directed to any point in the sky - it always looks only at the zenith. And to work with it, you will need an additional flat mirror, as they say, of optical quality.

Observing the Sun with our telescope can be done in the yard, on the balcony or even in the room by the open window. It is only necessary to fix a flat mirror over a rotating paraboloid so that the sun's rays fall on the mirror vertically or almost vertically. It is necessary to find the focal plane of the telescope in order to know at what distance to set the camera. This is done using a white screen onto which the image of the Sun is projected.

They take pictures from a rotating surface with a camera without a lens (its role is played by the liquid mirror itself), but with a hood that protects the film from side flare. Since the image can be unstable due to imperfect centering of the plate, fast shutter speeds must be used. A non-metallized flat mirror and a viscous liquid reflect only a few percent of the light falling on them, so you can not be afraid to burn the shutter doors or damage your eyesight.

The diameter of the image of the solar disk depends on the speed of rotation of the mirror. It can be approximately calculated by the formula:

d=405/n²

where d is in centimeters and n is in revolutions per minute.

It should be borne in mind that even a small change in the speed of rotation of a liquid mirror greatly changes its curvature. For example, at 33 min-1 (the speed of rotation of the player's disk), the focal length is about 40 cm (the diameter of the image of the Sun is only 4 mm), and at a speed of 10 min-1 it increases to 4,5 m. This is already quite acceptable for observing solar spots.

Author: V.Vasiliev

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