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。
Your body should be straight up in the water; don't lean forward much or hunch over. Keep your shoulders back and look straight ahead.
你的身体在水中应该保持正直，挺胸、目视前方，不要过度前倾或伛偻。
I keep telling the boys not to band doors but it's like water off a duck's back.
我一直对男孩们说不要呯地一声猛然关门，但是我的话尤如鸭背上的水，毫无作用。
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