🔦 光的直线传播：影子与小孔成像🔦 Rectilinear Propagation of Light: Shadows and Pinhole Images
上一片里，声音撞上墙壁会变成回声。那光呢？小火箭按下激光笔——光不会拐弯，也不会回声，只会笔直地往前冲。Last leaf, sound bounced off walls and came back as an echo. What about light? Our rocket clicks a laser pointer — light never bends around corners or echoes back. It just charges straight ahead.
第 1 步：影子和小孔里的倒像Step 1: Shadows and the upside-down pinhole image
光到底会不会拐弯？两个小实验就能看清。So does light ever bend? Two little experiments settle it.
光沿直线传播，不会拐弯绕开你的手，手后面就留下形状一样的影子。光穿过小孔时依旧走直线：上面的光跑到下面，下面的光跑到上面，在屏上拼出一个倒立的像——这就是小孔成像。像的大小由物距和像距一起决定，下面的游戏里可以亲手调。Light travels in straight lines: it cannot bend around your hand, so a hand-shaped shadow lands on the wall behind it. Through a tiny hole the rays still go straight — top rays land low, bottom rays land high — and the screen builds an upside-down image: the pinhole image. Its size is decided by both distances, and you can tune them in the game below.
影子的把戏只在小房间里有吗？抬头看看太阳。Are shadows only a small-room trick? Look up at the Sun.
第 2 步：日食——月球投下的影子Step 2: Solar eclipses — the Moon's shadow
太阳光笔直地奔向地球，月球挡在中间，就在地球表面投下一小片影子。站在影子里的人抬头看，太阳被月球遮住了——这就是日食。月食同理，只是这次轮到地球挡光，把影子投到了月球上。Sunlight races straight toward Earth; when the Moon stands in the way, it drops a small shadow onto Earth's surface. Inside that shadow, people look up and see the Sun hidden behind the Moon — a solar eclipse. A lunar eclipse works the same way, with Earth casting its shadow onto the Moon.
知道了光走直线，工程师们会怎么用？Once you know light stays straight, how do engineers use it?
第 3 步：让光替我们瞄准Step 3: Let the light do the aiming
挖隧道、铺管道时，工人先打出一束激光，让它在远处墙上落成一个小亮点，沿着这条笔直的光线开挖——这就是激光准直。测距、切割、手术定位也全靠它：光的直线，就是最可靠的尺子。When digging a tunnel or laying a pipe, workers fire a laser beam and let it land as a bright dot on the far wall, then follow that perfectly straight line — that's laser alignment. Ranging, cutting and surgical positioning all rely on it: a straight ray of light is the most reliable ruler there is.
🎮 你来当小孔实验员（1 分钟）🎮 Your turn: pinhole lab (1 minute)
道理讲完了。调一调物体高度和屏幕距离，让倒立的像达到目标大小，完成 4 个任务。Theory done. Adjust the object height and screen distance to hit each upside-down image-size target across 4 tasks.
一句话记住它：光在同种均匀介质中沿直线传播——影子、日食月食、小孔成像全是它的证据；激光准直是它在工程里的用法。Remember it in one line: in the same uniform medium light travels in straight lines — shadows, eclipses and pinhole images are all its evidence, and laser alignment is its everyday engineering use.
光走直线不绕弯：挡光就成影，影子形状和物体一样Straight rays cast shadows: block the light and the shadow copies the object's shape小孔成像：光交叉穿过小孔得到倒立的实像；像高 = 物高 × 像距 ÷ 物距Pinhole: rays cross at the hole and build an upside-down real image; h = H × v ÷ u日食月食是"天上的影子"；激光准直让直线变成瞄准线Eclipses are giant sky shadows; laser alignment turns straight lines into aiming lines
内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.
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