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Absorption in the visible region

Physical basis of our perception of color. Example of beta-carotene, the molecule that makes carrots orange. Created by Jay.

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Video transcript

On the right we have the dot structure for beta carotene which is an orange molecule that is responsible for the color of carrots. On the left is the absorption spectrum for beta carotene. And the reason why beta carotene has a color is because it absorbs light in the visible region of the electromagnetic spectrum. The visible region starts at approximately 400 nanometers, so if I draw a line right here, to the left of that line would be the Ultra-Violet, the UV region of the electromagnetic spectrum, and on the right would be the visible region. So we see that beta carotene absorbs light with wavelengths of approximately 450 to 500 nanometers for a range and it absorbs strongly in the visible region. To explain why beta carotene is orange, we need to look at a little bit more detail at the visible region of the electromagnetic spectrum, and so here we have the different colors. The colors in the visible region, so essentially the colors in the rainbow. Approximately 400 nanometers, we're talking about violet light, alright, so we have violet light right here, if we go beyond violet light, you're in the ultraviolet region or the UV region. The visible region goes to somewhere around 700 nanometers, or little bit beyond that, so when you're in 700 nanometers, you're talking about red light. And if you go just past red light you are in the infrared region of the electromagnetic spectrum. And here I have six colors, I have red, orange, yellow, green, blue, and violet, and when Isaac Newton did his famous experiment with a prism and he wrote down seven colors, he included indigo, because he wanted to have seven colors in the visible regions, so you usually memorize ROYGBIV for the colors of the rainbow. But the reason I've left out indigo here is because this allows us to better see a color wheel. Isaac Newton was the first to represent a color wheel, and you get a color wheel by taking the violet and moving it over here and taking the red and moving it over here, and so you put the violet right next to the red and so you get a color wheel. It's useful to look at a color wheel, because it allows you to see the relationship between complementary colors, for example, if I wanted to know the complementary color for red, all I have to do is look across on my color wheel and I can see that the complementary color is green. The complementary color for violet, if I look directly across, that would be yellow, and then finally, the complementary color for blue would be orange, and this is useful because it allows you to think about why things appear to be a certain color. For example, if I look at this orange sheet of paper here, and we try to understand why this sheet of paper is orange, we know that white light consists of all these different wavelengths, we know white light consists of all the different colors of the rainbow here. We could simplify that even further and we could think about white light being two complementary colors, so we can say oh, okay, so this part consists of blue wavelengths of light and then on the right here, this part of the color wheel consists of orange wavelengths of light and we can think about white light consisting of blue wavelengths and orange wavelengths so if we have white light coming in, here are the blue wavelengths of light, and then we have the orange wavelengths of light, so this is an oversimplified way to think about white light striking our orange object here. So if the object absorbs the blue wavelengths of light, so we are absorbing the blue wavelengths of light therefore we are reflecting the orange wavelengths. So if we reflect the orange wavelengths of light and our eye happens to be right here, we see the object as being orange. Our brains perceive the object as being orange because we are seeing the reflected orange light. And so that's how to think about why something appears to be a certain color. If I go back up here to beta carotene again, so I look and see where beta carotene is absorbing. Beta carotene is absorbing somewhere in the range of 450 to 500 nanometers and those are blue wavelengths of light, right, if I look at down here so 450 to 500 nanometers, we're absorbing the blue wavelengths of light. Therefore, we are reflecting the orange wavelengths. And so we perceive beta carotene to be orange. And so that's a little bit of the theory behind why we perceive something as having a certain color. In the next video we're going to talk about how this dot structure of beta carotene allows the molecule to be colored.