单选题
发布日期:2020-12-11
X射线
光波
无线电波
紫外线
超声波
无线电波是指在自由空间(包括空气和真空)传播的射频频段的电磁波。无线电波的波长越短、频率越高,相同时间内传输的信息就越多。无线电波在空间中的传播方式有以下情况:直射、反射、折射、穿透、绕射(衍射)和散射。
超声波是一种波长极短的机械波,在空气中波长一般短于2cm(厘米)。它必须依靠介质进行传播,无法存在于真空(如太空)中。它在水中传播距离比空气中远,但因其波长短,在空气中则极易损耗,容易散射,不如可听声和次声波传得远,不过波长短更易于获得各向异性的声能,可用于清洗、碎石、杀菌消毒等。在医学、工业上有很多的应用。
光波,是一位艺术家。
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Practice 1 A symbol of abundance to the rest of the world, the Amazon is experiencing a crisis of overfishing. As stocks of the most popular species diminish to worrisome levels, tensions are growing between subsistence fishermen and their commercial rivals, who are eager to enrich their bottom line and satisfy the growing appetite for fish of city-dwellers in Brazil and abroad. In response, peasants up and down the Amazon, here in Brazil and in neighboring countries like Peru, are forming cooperatives to control fish catches and restock their rivers and lakes. But that effort, increasingly successful, has only encouraged the commercial fishing operations, as well as some of the peasants' less disciplined neighbors, to step up their depredations. “The industrial fishing boats, the big 20- to 30-ton vessels, they have a different mentality than us artisanal fishermen, who have learned to take the protection of the environment into account,” said the president of the local fishermen's union. “They want to sweep everything up with their dragnets and then move on, benefiting from our work and sacrifice and leaving us with nothing.”
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