and that the color of an object

而物體的顏色取決於

is related to the frequency of light waves it reflects.

該物體反射出光線的頻率。

High-frequency light waves look violet,

高頻率的光波看起來是紫色，

low-frequency light waves look red,

低頻率的光波看起來是紅色，

and in-between frequencies look yellow,

而且兩者中間的頻率看起來可能是黃色、

green,

綠色、

orange,

橘色

and so on.

等顏色。

You might call this idea physical color

你可以說這是物理性的顏色，

because it says that color is a physical property of light itself.

因為它表示光的顏色源自於本身屬性。

It's not dependent on human perception.

和人類解讀能力沒有關係。

And, while this isn't wrong,

雖然這並非錯誤，

it isn't quite the whole story either.

但是不能完全解釋人如何辨別顏色。

For instance, you might have seen this picture before.

舉例而言，你可能看過這張圖。

As you can see, the region where the red and green lights overlap is yellow.

如你所見，紅色和綠色光疊加看起來是黃色。

When you think about it, this is pretty weird.

但如果你仔細思考，就會覺得怪怪的。

Because light is a wave, two different frequencies

因為光具波動性，兩種不同頻率的光波

shouldn't interact with each other at all,

不應該會有交互作用，

they should just co-exist

它們應該像是合唱演出般

like singers singing in harmony.

共同存在。

So, in this yellow looking region,

所以，在你看起來是黃色的區域

two different kinds of light waves are present:

有兩種光波存在：

one with a red frequency,

一種是紅光光波

and one with a green frequency.

另一種是綠光光波。

There is no yellow light present at all.

但這裡並無黃光光波的存在

So, how come this region,

那為什麼這個

where the red and green lights mix,

紅色和綠色疊加的區域

looks yellow to us?

看起來會是黃色呢？

To understand this, you have to understand a little bit about biology,

要瞭解這個現象，必須先學學生物學——

in particular, about how humans see color.

尤其是關於人如何看到顏色的部份。

Light perception happens in a paper-thin layer of cells,

光由人類眼球底部

called the retina,

薄薄一層

that covers the back of your eyeball.

被稱為視網膜的細胞所接收。

In the retina, there are two different types of light-detecting cells:

在視網膜上有兩種感光細胞

rods and cones.

桿狀細胞和錐狀細胞，

The rods are used for seeing in low-light conditions,

其中桿狀細胞在低亮度時 負責接收，

and there is only one kind of those.

而且只有一種。

The cones, however, are a different story.

而錐狀細胞就有所不同了！

There three kinds of cone cells that roughly correspond

我們有三種錐狀細胞，

to the colors red,

他們分別對應紅色、

green,

綠色、

and blue.

和藍色。

When you see a color,

當你看到顏色時，

each cone sends its own distinct signal to your brain.

不同的錐狀細胞會將不同的訊息送向腦部。

For example, suppose that yellow light,

舉例而言，

that is real yellow light, with a yellow frequency,

當有一道真正具有黃色頻率的光線

is shining on your eye.

射入你眼睛時，

You don't have a cone specifically for detecting yellow,

你並沒有專門負責接收黃光的錐狀細胞。

but yellow is kind of close to green

但是因為黃色和綠色

and also kind of close to red,

及紅色相近，

so both the red and green cones get activated,

所以負責紅色和綠色的錐狀細胞會被活化，

and each sends a signal to your brain saying so.

分別傳送訊息到你的腦部。

Of course, there is another way to activate

當然，有另一種方式可以達到相同效果：

the red cones and the green cones simultaneously:

紅色和綠色光同時抵達，

if both red light and green light are present at the same time.

造成負責紅色和綠色的錐狀細胞同時被啓動。

The point is, your brain receives the same signal,

這時候腦部會收到相同的訊號，

regardless of whether you see light that has the yellow frequency

不論是接受一束黃光

or light that is a mixture of the green and red frequencies.

或是由紅光和綠光所混成的光線都是一樣。

That's why, for light, red plus green equals yellow.

所以對光線來說，紅色加綠色就成了黃色。

And, how come you can't detect colors when it's dark?

那為何你無法在黑暗中辨別色彩呢？

Well, the rod cells in your retina take over

這是因為在低亮度的狀況下

in low-light conditions.

是由視網膜上的桿狀細胞接手感光。

You only have one kind of rod cell,

但是你只有一種桿狀細胞，

and so there is one type of signal

所以它只能送出一種訊號

that can get sent to your brain:

告訴你的大腦

light or no light.

是亮還是暗。

Having only one kind of light detector

因為只有一種偵測器，

doesn't leave any room for seeing color.

所以沒辦法偵測看到的顏色。

There are infinitely many different physical colors,

物理顏色近乎無限多種，

but, because we only have three kinds of cones,

而我們只有三種錐狀細胞，

the brain can be tricked into thinking it's seeing any color

大腦則藉由運算

by carefully adding together the right combination

紅、綠、藍

of just three colors:

這訊號強弱相加的結果

red, green, and blue.

讓我們看到不同色彩。

This property of human vision is really useful in the real world.

人類視覺原理在真實世界中十分有用。

For example, TV manufacturing.

例如：電視機的製造。

Instead of having to put infinitely many colors in your TV set

與其要放進無限種顏色的光源

to simulate the real world,

以模擬真實世界的色彩。

TV manufacturers only have to put three:

電視製造商只需要用到三種光源：

red, green, and blue,

紅、綠、藍。

which is lucky for them, really.

這對他們來說真是太幸運了！