Under proper conditions， sound waves will be reflected from a hillside① or other such obstruction②. Sound travels at the rate of about one-fifth of a mile per second. If the hill is eleven hundred feet away， it takes two seconds for the sound to travel to the hill and back. Thus， by timing the interval③ between a sound and its reflection（the echo）， you can estimate the distance to an obstruction.
During World War Ⅱthe British used a practical application of this principle to detect④ German planes on their way to bomb London long before the enemy was near the target. They used radio waves instead of sound waves， since radio waves can penetrate⑤ fog and clouds. The outnumbered⑥ Royal Air Force（RAF）always seemed to the puzzled Germans to be lying in wait at the right time and never to be surprised. It was radio echoes more than anything else that won the Battle of Britain.
Since the radio waves were used to tell the direction in which to send the RAF planes and the distance to send them（their range of flight， in other words）， the device was called radio directing and ranging， and from the initials⑦ the word radar was coined.
① hillsiden. 山腰，山坡[C]
② obstructionn. 阻碍，妨碍；阻塞[U]
③ intervaln. 间隔；距离[(+between)]
④ detectv. 发现，察觉；查出，看穿
⑤ penetratev. 穿过；刺入；透过
⑥ outnumberv. 数量上超过
⑦ initialn. (姓名或组织名称等的)首字母
“雷达”一词的创造
在适当条件下，声波会从山坡或其他类似的障碍物反射回来。声音以大约每秒1／5英里的速度传播。假如小山在1，100英尺之外，声音传到小山又反射回来需要两秒钟。这样，通过计算声音与其回声的间隔时间，你便能推算出某一障碍物有多远。
第二次世界大战期间，英国人实际运用了这一原理，使前去轰炸伦敦的德国飞机早在接近目标之前便被探测出来。他们使用的是无线电波，而不是声波，因为无线电波能穿透浓雾和云层。在感到迷惑的德国人看来，数量上处于劣势的皇家空军似乎总是适时地守候在那里，而从未被突然袭击过。不列颠之战的胜利主要是靠着无线电回声，而不是靠别的什么。
由于无线电波被用来测定派遣皇家空军飞机的方向和距离（或称航程范围），这一装置被称为”无线电定向和测距器”（radio directing and ranging），从（这几个字的）首字母产生了radar（雷达）一词。
