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神奇的激光

激光是20世纪的重大发明,被称为“最快的刀”、“最准的尺”和“最奇异的光”.

激光是一种颜色单纯的光,激光器发出的光通过三梭镜后不会被分成不同颜色的光.

激光具有超强的能量密度,可以在直径几百纳米的范围内产生几百万度的高温.工业上 ,激光刀可切割坚硬和柔软的材料,对切割部位进行局部照射几秒钟便能切割好.激光刀还是外科医生的好帮手,用它做手术没有丝毫机械撞击,而且又快又精准,大大减轻了病人的痛苦,如血管瘤、脑部手术等.

激光单色性好、方向性强、测距精度高,使得测量人造卫星、月球等远目标的距离变成现实.宇航员在月球上安放激光反射器,激光器从地球向月球发射一个激光脉冲信号,月球上的反射器能够将激光脉冲原路返回,激光脉冲从发出到接收时间间隔为2.56s.

激光也被作为电影素材,在科幻电影《星球大战》中天行者使用 的激光剑能释放出一道长约 l m的纯能量束,战斗时悄无声息.

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阅读下列短文,从下面每小题的A、B、C、D四个选项中选出最佳选项。

    There is always a shadow of shopping in life. You like this, look at that, and then buy a lot of things and go home. Then you calm down and find out that the shopping doesn't make any sense but waste money. How do you avoid the wrong shopping? I'll give you a few tips today!

⑴Buy good quality — and use it

    It's common for people to almost never use the things they love the most. Why? Often, it's because they want to protect the item. They like it so much and don't want it to be ruined. Instead of using their favourites, they buy much cheaper things for "everyday" use. The smarter way is to put your money on something with good quality and good style that you love very much and enjoy it the most. It stops you from wasting money on unnecessary things.

⑵Play the waiting game

    Before you decide to pay for something you like very much at first sight, wait for at least 20 minutes. Then, after clearing your head, think about how and when you'll actually use the product. Instead of simply choosing to have it or not have it, think for a moment and ask yourself whether there is something else even more important you should buy. It reminds you to save money.

⑶Buy only what you need, right now

    Besides, what makes shopping so alluring (欲罢不能的) is your idea that comes with imagining how a product can be used, such as "I will look like a princess in the evening dress at the ball one day." Before paying for a product, ask yourself if you will use it in the very near future. If the answer is no or not likely, pass.

⑷Buy for the right reason

    Research shows that we may think we're hungry when we are actually thirsty, and think we are tired when we are bored in reality. In other words, we know when we need something to satisfy ourselves, but we're not good at realizing what we truly need. It is smart to always ask yourself why you need the product before you pay for it.