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Shadows are formed by light shining on objects. Shadows are different everywhere on the earth. Carefully observe the shadow under the light of the electric light, you can also find that the shadow is particularly dark in the middle and slightly shallow around. The dark part in the middle of the shadow is called the umbra, and the dark part around it is called penumbra. These phenomena are closely related to the linear propagation of light. If you put a columnar tea tube on the table and light a candle next to it, it will cast a clear shadow. If you light two candles next to the tea tube, they will form two overlapping but not overlapping shadows. There is no light on the overlapped part of the shadow, which is completely black. This is the umbra; the penumbra is half dark where only one candle can reach it. If you light three or even four candles, the umbra will gradually shrink and the penumbra will appear in many levels. This is also true of the fact that objects can produce shadows composed of umbra and penumbra under electric light. Obviously, the larger the area of the luminous object, the smaller the umbra. If we light a circle of candles around the above-mentioned tea pot, the umbra disappears completely and the penumbra is too faint to be seen. According to the above principle, scientists made shadowless lamp for operation. It arranges the lamps with high luminous intensity into a circle on the lamp panel to synthesize a large area light source. In this way, we can irradiate the light to the operating table from different angles, which not only ensures that the surgical field of vision has sufficient brightness, but also does not produce obvious umbra, so it is called shadowless lamp.