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1998年抗洪中全国共牺牲多少民兵？
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Invisibility Ring
Scientists can"t yet make an invisibility cloak like the one that Harry Potter uses. But， for the first time， they"ve constructed a simple cloaking device that makes itself and something placed inside it invisible to microwaves.
When a person "sees" an object， his or her eye senses many different waves of visible light as they bounce off the object. The eye and brain then work together to organize the sensations and reconstruct the object"s original shape. So， to make an object invisible， scientists have to keep waves from bouncing off it. And they have to make sure the object casts no shadow. Otherwise， the absence of reflected light on one side would give the object away.
Invisibility isn"t possible yet with waves of light that the human eye can see. But it is now possible with microwaves. Like visible light， microwaves are a form. of radiant energy. They are part of the electromagnetic spectrum， which also includes radio waves， infrared light， ultravioletmys， X rays， and gamma rays. The wave lengths of microwaves are shorter than those of radio waves but longer than those of visible light.
The scientists" new "invisibility device" is the size of a drink coaster and shaped likearing. The ring is made of a special material with unusual ability. When microwaves strike thering， very few bounce off it. Instead， they pass through the ring， which bends the waves all the way around until they reach the opposite side. The waves then return to their original paths.
To a detector set up to receive microwaves on the other side of the ring， it looks as if the waves never changed their paths as if there were no object in the way! So， the ring is effectively invisible.
When the researchers put a small copper loop inside the ring， it， too， is nearly invisible.
However， the cloaking device and anything inside it do cast a pale shadow. And the device works only for microwaves， not for visible light or any kind of electromagnetic radiation. So， Harry Potter"s invisibility cloak doesn"t have any real competition yet.
Harry Potter is mentioned in the passage， because scientists __________. 查看材料
A .利润
B .支出
C .收入
D .债务
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