High-frequency light waves look violet, low-frequency light waves look red, and in-between frequencies look yellow, green, orange, and so on.

高頻率的光波看起來是紫色，低頻率的看起來是紅色，而介於兩者間的頻率看起來是黃色、綠色、橘色等。

You might call this idea physical color because it says that color is a physical property of light itself.

可以稱之為「物理顏色」，意思是顏色就是光本身的物理屬性。

It's not dependent on human perception.

和人類解讀能力沒有關係。

And, while this isn't wrong, it isn't quite the whole story either.

雖然這並非錯誤，但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，紅色和綠色光疊加起來是黃色。

When you think about it, this is pretty weird.

但如果仔細思考，就會覺得怪怪的。

Because light is a wave, two different frequencies shouldn't interact with each other at all, they should just co-exist like singers singing in harmony.

既然光是一種波，兩種不同頻率的光波應該不會交互作用，而是會像合唱演出般共同存在。

So, in this yellow looking region, two different kinds of light waves are present.

所以，在這個看起來像是黃色的區域中會有兩種光波存在：

One with a red frequency, and one with a green frequency.

紅光波和綠光波。

There is no yellow light present at all.

沒有黃光這種東西。

So, how come this region, where the red and green lights mix, looks yellow to us?

那為什麼紅色和綠色疊加的區域看起來會是黃色呢？

To understand this, you have to understand a little bit about biology, in particular, about how humans see color.

要瞭解這個現象，必須先知曉生物學——尤其關於人如何看到顏色的部份。

Light perception happens in a paper-thin layer of cells, called the retina, that covers the back of your eyeball.

光是由人類眼球底部薄薄一層，被稱為視網膜的細胞所接收。

In the retina, there are two different types of light-detecting cells: rods and cones.

視網膜上有兩種感光細胞：桿狀細胞和錐狀細胞。

The rods are used for seeing in low-light conditions, and there is only one kind of those.

其中桿狀細胞在低亮度時負責接收光，而且只能接收一種。

The cones, however, are a different story.

而錐狀細胞就大不相同了。

There are three kinds of cone cells that roughly correspond to the colors red, green, and blue.

有三種錐狀細胞，分別對應紅、綠和藍。

When you see a color, each cone sends its own distinct signal to your brain.

當你看見顏色時，不同的錐狀細胞會將不同的訊息傳送至腦部。

For example, suppose that yellow light, that is real yellow light, with a yellow frequency, is shining on your eye.

舉例來說，當一道黃色頻率的光線射入眼睛時，

You don't have a cone specifically for detecting yellow, but yellow is kind of close to green and also kind of close to red,

你並沒有專門接收黃光的錐狀細胞，但因為黃色和綠色及紅色相近，

so both the red and green cones get activated, and each sends a signal to your brain saying so.

所以負責紅色和綠色的錐狀細胞會被激活，並分別傳送訊息到腦部。

Of course, there is another way to activate the red cones and the green cones simultaneously.

當然，還有另一種方式可以達到相同效果：

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

那就是紅色和綠色光同時抵達，同時觸發兩者錐狀細胞。

The point is, your brain receives the same signal, regardless of whether you see light that has the yellow frequency or light that is a mixture of the green and red frequencies.

這時腦部會收到相同訊號，不論是接受一束黃光或是由紅光和綠光所混成的光線。

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

所以就光線而言，紅加綠就成了黃色。

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

那為何人無法在黑暗中辨別色彩呢？

Well, the rod cells in your retina take over in low-light conditions.

這是因為低亮度的狀況下，是視網膜上的桿狀細胞在感光。

You only have one kind of rod cell, and so there is one type of signal that can get sent to your brain: light or no light.

但人類只有一種桿狀細胞，所以只能送出一種訊號到大腦。那就是有光還是沒光。

Having only one kind of light detector doesn't leave any room for seeing color.

因為只有一種感測器，所以沒辦法看到顏色。

There are infinitely many different physical colors, but because we only have three kinds of cones,

物理顏色近乎無限多種，而我們只有三種錐狀細胞。

the brain can be tricked into thinking it's seeing any color by carefully adding together the right combination of just three colors: red, green, and blue.

大腦藉由運算紅、綠、藍三種訊號強弱相加的結果，讓我們看到不同色彩。

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

人類視覺原理在現實中十分有用。

For example, TV manufacturing.

例如：電視機的製造。

Instead of having to put infinitely many colors in your TV set to simulate the real world, TV manufacturers only have to put three:

與其放無限種色彩的光源模擬真實世界色彩，電視製造商只需使用三種光源：

red, green, and blue, which is lucky for them, really.

紅、綠和藍，這對他們來說真是幸是一件！