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How does gold get its color?
Question Date: 2014-02-08
Answer 1:

Great question! Why is that gold is… well gold, while most other metals, like aluminum, are grey? This is a complicated question that has a very complicated answer, but I'll try to explain the main concept.

Let's begin by reminding ourselves that white light is composed of all the different colors of light put together: red, orange, yellow, green, blue, indigo, violet, and every color in between. When sunlight reaches the Earth it is composed of all of these colors; in other words, sunlight is white light. When the light hits an object, the object can either absorb the light or not absorb the light. In general, the absorption of different colors is not equal, meaning that an object will absorb specific colors of light and not absorb others.

When sunlight interacts with a piece of gold, the gold tends to absorb blue light and not absorb green and red light; instead, the green and red light is partially reflected. What we as humans see is the combination of the reflected green and red light, which gives the classic golden yellow color!

In other metals, like aluminum, the colors of white light are almost equally absorbed and reflected and thus what we see is a silvery, white-ish material!

Hope this helps you understand the basic premise!

Bonus question: Using this same concept, what color of light do most plants absorb and not absorb?

Bonus answer: Most plants are green and thus they tend to absorb all the colors but green and not absorb green (i.e. the green color is reflected, which is what we see!)

Answer 2:

Gold is a type of material known as a metal. Most metals reflect (bounce back) the visible (white) light that is shined onto their surfaces. A good example is the metal silver, which is used in mirrors. The reflection of visible light off of the surface of silver is the reason that you can see yourself when looking into a mirror.

Gold is a special metal that both reflects (bounces back) and absorbs ("keeps") some of the visible (white) light that is shined onto its surface. An important idea is that visible light is called white light because it is made up of all of the different colors of a rainbow: red, orange, yellow, green, blue, indigo, violet, which combine together to produce the color white. See the following webpage for a picture of a rainbow: Rainbow picture.

It turns out that the colors that are absorbed ("kept") by gold are the darker colors of green, blue, indigo, and violet (the bottom part of the rainbow), while the colors that are reflected (bounced back) by gold are red, orange, and yellow (the top part of the rainbow). When these bounced back, reflected colors all reach your eye, they blend to end up giving gold the shiny, yellow appearance that is so familiar (as long as the gold is clean and polished!).

Thanks for your great question, and please feel free to send more excellent questions our way!


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