If the two media have really different densities, like air and water, then most of the energy is going to be reflected back, very little of it will keep going and get transmitted into the new medium.
而如果这两种介质的密度相差较大，就像空气和水那样，那么大部分的能量将会折射回来，只有一小部分会继续传播进入到新的介质中。
I will give you some land and the old house in the back of the ranch if you keep the horse there.
如果你让这匹马在那儿，我将把一些土地和牧场后边的那座老房子给你。
Barricades were erected to keep back the crowds.
设置了障碍，使人群无法靠近。
It is true in life that if we just keep going forward without looking back, we are more likely to reach our goals.
在生活中，如果我们只是勇往直前而不回头，我们更有可能实现我们的目标。
To help generate lift to keep midwater, the top of the tail extends farther back than the bottom, creating a slant along the back edge.
为了保持在水中不至于沉底，尾部的顶端比底端向后延伸，沿着后端形成一个斜面，产生提力。
For men, one of the best places to keep a wallet is in the back pocket of tight trousers.
对于男性来说，存放钱包的最佳位置之一是紧身裤后面的口袋。
They believe that the best we can do is keep it at a reasonable level, and at present the only serious option for doing this is cutting back on our carbon emissions.
他们认为，我们所能做的最好的事情就是把它控制在一个合理的水平，而在这方面目前唯一重要的选择就是减少碳排放。
Identify the kinds of interests they keep coming back to, as these offer clues to the careers that will fit them best.
要确定他们能够坚持的兴趣爱好，因为这些爱好暗示了什么样的职业最适合他们。
To keep the astronauts and the spacecraft safe, the astronauts must leave and go back to the spacecraft through a special door.
为了保证宇航员和飞船的安全，宇航员必须通过一扇特殊的门离开和返回飞船。
Tom saw his opportunity, "Lookyhere, Huck, being rich ain't going to keep me back from turning robber."
汤姆看到了他机会，说：“喂，哈克，有钱也阻止不了我当强盗。”
He was moving back and forth to keep warm and asking for change very softly.
他来回走动取暖，轻声地要些零钱。
Even today when Will thinks that he won't be able to do something, he will look back to this experience and keep telling himself, "one brick at a time".
即使在今天，当威尔认为自己做不到一件事时，他会回顾这段经历，并不断告诉自己：“一次添一块砖”。
My mother was so touched by her gesture that she decided to go back to the store and give the cashier a five-dollar bill to keep on hand in case the same happened to someone else if they didn't have enough money for all of their groceries.
我母亲被她的举动感动了，她决定回到店里，给收银员一张五美元的钞票，让他放在手边，以防别人在没有足够的钱支付所购买的东西时发生同样的情况。
Voters “keep sending me back regardless of what I say,” he chuckled.
“无论我说了什么，选民还总是把我选回来”他打趣道。
Keep going back to these facts and show yourself there's no rational basis for your fear.
一直回想这样的事实，告诉自己你的恐惧其实是没有合理依据的。
We liked each other's voices – each other's imaginations – enough to keep calling back.
足以让我们一直保持联系的是，我们都很迷恋对方的声音和彼此想象的场景。
Perhaps I keep circling back to grace because it is one grand theological word that has not spoiled.
我一直在说恩典，也许是因为这还是一个很伟大的词汇，还没有被破坏。
I keep coming back to "consciousness" as the cornerstone of most effective learning habits.
我一直反复强调自觉意识是一切高效学习习惯的基石。
When we practice mindfulness we keep calling back our wandering mind to rest on the current moment.
练习正念时，我们要一直不停地唤回游走的思绪，将其定格在当下时刻。
The man keep going back and forth in front of me which make me really annoyed.
那人在我跟前来来回回不停的晃悠，弄的我很恼火！
Millers tried to keep supplies back, waiting for higher prices.
农场主试图将供应的大米囤积，等候更高的价格。
But it won't go away for good. It'll keep coming back... for decades.
也许在几个星期或几个月以后它会消失，但是它不会永远消失，还会不断地回来……达数十年之久。
Martin shook his head, but he had failed to keep back the wolfish, hungry look that leapt into his eyes at the suggestion of dinner.
马丁摇摇头，但是他听见吃饭时眼里所流露出的饿狼般的馋相，却无法掩饰。
Plants that keep growing back are called perennials.
能生长很多年的植物叫perennial（多年生植物）。
At the same time, watch any vehicles in front of you on the ramp and keep back a safe distance.
同时，注意观察你前方同在匝道上驶行车辆并保持安全的距离。
She struggled to keep back her tears.
她努力地忍住泪水。
She struggled to keep back her tears.
她努力地忍住泪水。
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