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气体中光的折射

Refraction in Gases

[音乐播放]
[MUSIC PLAYING]
你有想过光是怎样穿过
Have you ever wondered how light travels
不同的物质到达我们的眼睛的吗?
through different materials to our eyes?
你看看这个花瓶 光线弯曲 所以
When you look through this vase, the light bends, and so
你所看到的是扭曲的形状
the image you see is distorted.
这种现象称为折射
This phenomenon is called refraction.
我们身边每时每刻都有这种现象 例如
It’s all around us every day, like when
你戴眼镜看书时或者当你透过一杯水
you read through your glasses, or when you look at something
看东西时
through a glass of water.
你会发现 钢笔在水中 看起来会弯曲
As you can see, the pen appears to bend as it enters the water.
折射是指 当光线进入不同的透明物质时
Refraction is the slowing down and bending of light
它的前进速度放慢 前进方向发生改变的现象
as it moves through different transparent substances.
让我们看看 当光波穿过两种不同物质的边界时
Let’s take a look at what happens to light waves
会发生什么情况
as they cross the border between two different materials.
现在这种情况 顶部材料是空气
We’ll say for now that the material on top is air,
底部材料是玻璃
and the material on the bottom is glass.
你会发现 光波在玻璃中传播的速度
You can see that the light waves traveling in the glass
比在空气中慢很多
are moving slower than those in the air.
材料的属性–折射率
The material property that causes this difference
是引起这种差异的原因
is the index of refraction.
我们可以这样理解
We define this as the speed of light
相对于 光在真空中的速度 它在介质中的速度产生了分裂
in a vacuum divided by the speed of light in the material.
光和物质的相互作用越大
The more the light interacts with the material,
它传播得速度就越慢 折射率也就越高
the slower it travels, and the higher the index of refraction.
例如 玻璃的折射率
For example, the index of refraction of glass
大约为1.5 也就是说光速
is about 1.5, meaning light travels 1.5 times
在真空中的速度比在玻璃中快1.5倍
slower in glass than in a vacuum.
当光穿过两种不同物质的边界时
As light crosses the boundary between two materials,
方向也会改变
the direction also changes.
走在前面的光波首先进入媒介材料时
The portion of the wave front that enters
速度会放慢 导致射线方向弯曲
first slows down, causing the ray to bend in that direction.
从本质上来说 光线似乎想
Essentially, light acts as if it wants
在折射率高的地方传播
to go to the area of higher index,
或者是想在两点之间选择更快的路径
or to take the quickest path between two points.
拿海市蜃楼现象来举个例子
Take the phenomenon of a mirage.
当你在地平线上看到一些事物时
This is when you see something on the horizon that
其实并不在同一平面上 因为光线
isn’t actually there, because the light has
是从其它平面照到你眼睛里的
traveled from beyond the horizon to meet your eye.
之所以有海市蜃楼现象 是因为靠近地面的空气
In a mirage, the air close to the ground
会升温 扩张 其折射率会降低
heats up and expands, reducing the index of refraction.
贴近地面的光波传播时
The light waves passing close to the ground
向上弯曲 事物的映像便看起来像在湖面上一样
bend upwards and look like reflections in a lake.
我们可以通过添加一层盐在缸的底部
We can make a similar situation in water
在水中做一个相似的情景
by adding a layer of brine in the bottom of a tank.
对于这个实验 我们需要水 盐 一个缸
For this experiment, we’ll need water, salt, a tank,
和激光
and a laser.
盐水的折射率
The index of refraction of the brine
高于淡水的折射率
is higher than that of the fresh water
与冷空气的折射率一样
and acts in the same way as the cold air [INAUDIBLE].
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[MUSIC PLAYING]
我们正用一个漏斗把盐溶液放到
We’re using a funnel to put the salt solution
水槽的底部
at the bottom of the tank.
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[MUSIC PLAYING]
我们把水槽放一会儿
We let the tank sit for a while to allow
使得淡水和盐水稍微混合
the fresh water and the salty water to mix a little.
你会发现盐的浓度
You can see that the concentration of salt
在边界处发生改变 同时
changes across the boundary, and so does
折射率也在变化
the index of refraction.
盐越多 越难
The more salt there is, the harder
光线运动起来就越困难
it is for the light to move forward.
因此折射率也就越高
And thus the higher the refractive index.
现在 让我们看看当灯关了 打开激光以后
Now let’s see what happens when we turn off the light
会发生什么
and turn on the laser.
当照射水槽底部 光线便会弯曲
As you go deeper into the tank, the light bends,
我们就创造出一个小型的水中海市蜃楼
and we’ve created a kitchen-sized, watery mirage.
你可能会注意到 这个实验中的光线
You might notice that the light in this case
是向下弯曲的 正好和
is bending down, opposite to what
你所期待的海市蜃楼的弯曲方向相反
you would expect in a mirage.
这是因为高折射率的材料
This is because the higher refractive index material
在缸的底部
is on the bottom of the tank.
现在 这是相当明显地在放大
Now, this is pretty obvious in things like magnifying
镜头和水
lenses and water.
但是光线弯曲也存在于我们周围的空气等气体中吗
But does light bend in gases like the air around us?
简单来说 是的
The short answer is yes.
复杂点说 只要气体中的温度 组成成分
The long answer is yes, but only if something in the gas
等其它条件发生变化时 光线就会弯曲
changes, like its temperature or its composition.
事实上 在海边 这被称为海市蜃楼
In nature, on the seashore, it is called a Fata Morgana,
或者倒海市蜃楼 看起来令人印象深刻
or inverted mirage, and can look pretty impressive.
这关键在于水面上
Its origin lies in the humidity and temperature of air
空气的湿度和温度
above the water.
贴近海平面的冷空气
The cold air is denser, close to the sea level,
比海平面上方的空气浓度大
then high above it.
这就产生了一个密度梯度
This creates a density gradient, and that’s
就像我们实验一样 折射率发生了变化
a change of refractive index, just like in our experiment.
来自船上的光线 其运行方向在空气中弯曲
The light coming from the boat is curved by the air layers.
但是我们的眼睛看到的却是
But our eyes think that the light
光线仍然是按直线传播的
travels in straight lines.
所以就好像是 我们看到了飞船
So it looks like we’ve seen a flying boat.
所有这些光线弯曲的例子
All of these examples of light bending
都是因为光线的折射
are caused by the same thing, refraction.
在实验室中 我们可以看到的海市蜃楼只有几英尺长
In the lab, we can make foot-long mirages.
但在野外 这长度范围
But in the wild, the length scale
会长达几英里
is more like several miles.
所以下次如果你在沙漠中迷路或者看到一艘飞船
So next time you get lost in the desert or see a flying ship,
不要相信你的眼睛
don’t trust your eyes.
相信这只是折射
Trust refraction.
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[MUSIC PLAYING]

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视频来源

https://www.youtube.com/watch?v=4AwNFxPFQBM

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