Completely black body. Fun physical experience at home

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Completely black body. Physical experiments

Entertaining experiments in physics

Entertaining experiences at home / Physics experiments for children

When they say about a black object: "a black object" is a wrong expression. Black does not exist. The word "color" does not apply to black. Blackness is the absence of light. But in colloquial language, we used to call black a color.

In physics there is a concept - "absolutely black body". This refers to a body that does not reflect the rays falling on it at all. There is no such body in nature, but scientists have created it artificially. And we can make a model that will give some idea of the "absolutely black body".

At home there is always some small cardboard box or perfume case, preferably cylindrical. The box should have a tight lid. Paint the box inside and out with black (non-shiny) paint so that it dries to a black matte finish. Then, in its end part, in the middle of its cover, make a small hole, the size of a penny coin. This hole will give the impression of a "pure black body" - it will appear blacker than the blackest outer areas of the box. (In the figure, for clarity, the body of the box is not blackened.)

Completely black body

Why does it work? A beam of light, entering this hole, falls on the inner wall and is reflected from it. A very small part of the beam will be reflected, since the inner wall is black and it will absorb most of the beam. The reflected part of the beam hits another black wall inside the box and is reflected again. This goes on many times, with the beam getting weaker each time. And there is nothing to go outside at all. Therefore, the hole looks completely black.

Experiments with "absolutely black body" scientists attach great importance. They study the ability of various substances to absorb and emit thermal and light energy. For scientific experiments, a metal cylinder with a hole is made. The cylinder is very hot, and special thermometers show the heating temperature. The hole in the cylinder is the "absolutely black body". When cold, it best absorbs light rays, and when the cylinder is very hot, the hole best emits both light and heat rays.

Black objects surrounding us absorb heat rays much more strongly than light ones. In the chapter on heat we will do some experiments with the absorption and emission of heat.

Author: Rabiza F.V.

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Bioengineers have used rice grains as a base for growing beef to enrich the plant product with the nutritional elements of meat. Currently, rice contains slightly more fat and protein, but in the future it is planned to increase the number of cells, which will increase these indicators. The cost of this rice is estimated at about $2,23 per kilogram, compared to regular rice at $2,2, but significantly cheaper than beef at $14,88.

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