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#### 气体中光的折射

Refraction in Gases

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Have you ever wondered how light travels

through different materials to our eyes?

When you look through this vase, the light bends, and so

the image you see is distorted.

This phenomenon is called refraction.

It’s all around us every day, like when

through a glass of water.

As you can see, the pen appears to bend as it enters the water.

Refraction is the slowing down and bending of light

as it moves through different transparent substances.

Let’s take a look at what happens to light waves

as they cross the border between two different materials.

We’ll say for now that the material on top is air,

and the material on the bottom is glass.

You can see that the light waves traveling in the glass

are moving slower than those in the air.

The material property that causes this difference

is the index of refraction.

We define this as the speed of light

in a vacuum divided by the speed of light in the material.

The more the light interacts with the material,

the slower it travels, and the higher the index of refraction.

For example, the index of refraction of glass

is about 1.5, meaning light travels 1.5 times

slower in glass than in a vacuum.

As light crosses the boundary between two materials,

the direction also changes.

The portion of the wave front that enters

first slows down, causing the ray to bend in that direction.

Essentially, light acts as if it wants

to go to the area of higher index,

or to take the quickest path between two points.

Take the phenomenon of a mirage.

This is when you see something on the horizon that

isn’t actually there, because the light has

traveled from beyond the horizon to meet your eye.

In a mirage, the air close to the ground

heats up and expands, reducing the index of refraction.

The light waves passing close to the ground

bend upwards and look like reflections in a lake.

We can make a similar situation in water

by adding a layer of brine in the bottom of a tank.

For this experiment, we’ll need water, salt, a tank,

and a laser.

The index of refraction of the brine

is higher than that of the fresh water

and acts in the same way as the cold air [INAUDIBLE].
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We’re using a funnel to put the salt solution

at the bottom of the tank.
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We let the tank sit for a while to allow

the fresh water and the salty water to mix a little.

You can see that the concentration of salt

changes across the boundary, and so does

the index of refraction.

The more salt there is, the harder

it is for the light to move forward.

And thus the higher the refractive index.

Now let’s see what happens when we turn off the light

and turn on the laser.

As you go deeper into the tank, the light bends,

and we’ve created a kitchen-sized, watery mirage.

You might notice that the light in this case

is bending down, opposite to what

you would expect in a mirage.

This is because the higher refractive index material

is on the bottom of the tank.

Now, this is pretty obvious in things like magnifying

lenses and water.

But does light bend in gases like the air around us?

The long answer is yes, but only if something in the gas

changes, like its temperature or its composition.

In nature, on the seashore, it is called a Fata Morgana,

or inverted mirage, and can look pretty impressive.

Its origin lies in the humidity and temperature of air

above the water.

The cold air is denser, close to the sea level,

then high above it.

This creates a density gradient, and that’s

a change of refractive index, just like in our experiment.

The light coming from the boat is curved by the air layers.

But our eyes think that the light

travels in straight lines.

So it looks like we’ve seen a flying boat.

All of these examples of light bending

are caused by the same thing, refraction.

In the lab, we can make foot-long mirages.

But in the wild, the length scale

is more like several miles.

So next time you get lost in the desert or see a flying ship,

Trust refraction.
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