Objects will have shadows under the sun, and there will also be shadows under the lights. Using shadows, you can measure the height of chess poles, measure the height of buildings, etc. It can be seen that the use of shadows is great, but sometimes people do not need shadows, such as classmates. When we do homework under the light, if the shadow shines on the workbook, it will affect the eyes and cause myopia; when the doctor performs surgery under the light, if there is a shadow on the operating table, it will affect the doctor's operation, and so on. Is there any way to make people have no shadow under the light? Therefore, people invented the "shadowless lamp".

Does "Shadowless Lamp" really produce no shadows? This has to start with the relationship between light and shadow. Due to the linear propagation of light, when light hits an opaque object, a shadow will be formed behind the object, and the shadow will be different in different places and at different times.

For example, the shadow of the same person under the sun is long in the morning and evening and short in the noon. By observing, we can find that the shadow of the object under the electric light is particularly dark in the middle and slightly lighter around. The dark part in the middle of the shadow is called the umbra, and the dark part around it is called the penumbra. The generation of these phenomena is closely related to the principle of linear propagation of the light. The mystery can be known through the following experiment.

We put an opaque cup on a horizontal table, and light a candle next to it, and a clear shadow will be cast behind the cup. If two candles are lit next to the cup, two overlapping and non-overlapping shadows will be formed, the overlapping part of the two shadows has no light at all, so it is completely black. This is the umbra: the place where only one candle can be found next to the umbra is the half-light and half-dark penumbra. If you light three or even four or more candles, the umbra part will gradually shrink, and the penumbra part will appear many layers and gradually become darker. Objects can produce shadows composed of umbra and penumbra under electric light.

The same is true. Electric lamps have a curved filament that emits light, and the luminous point is not limited to a point where the light from a certain point is blocked by the object, and the light from other points is not necessarily blocked. Obviously, the larger area of the luminous body, the smaller umbra. If we light a circle of candles around the above cup, then the umbra disappears, and the penumbra is too faint to be seen. For this reason, the ceiling light in the operating room is composed of several lamps with high luminous intensity. A circular illuminator with a diameter of about 1 meter is arranged on the disk, and a large area of light source can be synthesized so that the light can be illuminated on the operating table from different angles. So it avoid the umbra affecting the operation, which is convenient for the doctor to complete the operation smoothly. Therefore, it is named "shadowless lamp".