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GulliblePlantain6572 t1_jcnyt2w wrote

Reply to comment by Greyswandir in What decides which wavelength to reflect and which one to be absorbed in an object? by TheSimpleHumans

What I'm confused about is why are some absorption spectra shown as graphs with a continuous line? I was under the impression that only specific light with one or more specific energy, frequency, and wavelength could be absorbed by a given atom/molecule. Also, how do we find what color something appears to be from it's maximum absorption? For example, water absorbs red more than other visible light, so it's absorption maximum that we can see is red. How do we know from this that water is blue? I know there are complementary colors but I'm confused on how we actually got those. I made a post here recently asking basically this but it hasn't been put up yet.

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Greyswandir t1_jco01rd wrote

Like I said in my final paragraph, I was glossing over a lot :P

So any given molecule has a lot of energy levels. And for each of those transitions, there is actually a narrow range of acceptable wavelengths that will be absorbed. Start adding all of those together and what you end up with is that pretty much any wavelength of light can be absorbed, some are just much more strongly absorbed than others.

So an absorption spectrum is showing how strongly light is absorbed. The higher on the y axis, the more of that light gets absorbed by the material and the less is available to reach your eye.

Looking at water, it strongly absorbs UV (high energy light) and then there is a big drop and it has a minima right around 420 nm, which is blue. From there are the wavelength gets longer (redder) the graph ticks back up until it passes into the IR. So this tells us that water passes blue light and absorbs red light. So if you shine a red light through water, it will go away much faster than if you shine a blue light. From the graph we can (successfully) guess that water will appear blue or blue-green because it more easily allows blue light to pass (and reach our eyes) than it does yellow/orange/red light.

Complementary colors have more to do with how we perceive color than with how light works. It’s about which colors look good together rather than how those colors are made.

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GulliblePlantain6572 t1_jco58ix wrote

Thank you for elaborating! So the color of something is mainly based on what color of those that we can see that is absorbed least, pretty intuitive. I'm assuming there are also times when we would have to take combinations into account if there are a few wavelengths that are not absorbed much? Also I was reading a bit more about complementary colors and I think the premise is that 2 complementary colors would combine to form white. So I guess the idea is that if one color is absorbed much more than others, we would interpret the remaining light as the complementary color to the one that was absorbed, since the light was initially white and lost a lot of light of a specific color.

And 1 more question, does an emission spectrum for some molecule give essentially the same information as an absorption spectrum?

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Greyswandir t1_jcq5ask wrote

Key distinction: the color of something is based on the spectrum of light (how much light at each wavelength in the visible range) reaches our eye. Absorption is a big part in that, but it’s not all of it. For example: an apple won’t appear red if the light illuminating it doesn’t include any red light since there’s no red light there to reach our eyes. And there are plenty of situations where optical effects other than absorption are dominant. A great practical example is the Lycurgus Cup. It’s a glass cup full of gold nano particles. When light shines through it, absorption dominates and so only red light passes through and it appears red. When light shines on it, scattering dominates, and those same particles scatter green light, but not red or blue, so it appears green since the scattered green light is reaching our eye.

Also, up until now we’ve been talking about the light itself. How we perceive that light and turn it into our sense of color is a whole other part of this. Very simplified explanation but: our eyes contain two types of light sensitive cells, rods and cones. Rods are great in low light conditions but can only see black and white. Cones are less sensitive to light, but they come in three variants, red, blue, and green. Red cones most strongly absorb red light, blue cones most strongly absorb blue and green absorbs green. When a cone absorbs light, it sends a signal to our brain. Based on how many of the RGB cones are triggered our brain mixes those signals together to form a perceived color. So complimentary colors have to do with how our eye and brain perceive colors. I don’t think there’s anything inherent to the photons that makes, say, blue the complement of red. It’s the way our sense of color works that makes those have high contrast.

Emission specta (or scattering spectra, or transmission spectra, etc) are basically the same yeah. You read the wavelength (or frequency) of light along the x axis and the y value tells you how much is emitted/scattered etc for that wavelength. It’s been a while but I think for an emission spectrum you need to specify the conditions under which the object is emitting.

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GulliblePlantain6572 t1_jcquqiw wrote

How do you shine light on something vs through something? And I agree that the whole complementary color thing doesn't tell us much about the light inherently but it still seems useful (or at least interesting) to determine what color we perceive things to be.

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Greyswandir t1_jcqy9z6 wrote

Basically is it backlit or lit from the front. If you backlight the cup you are looking at the light that has passed from the light source through the cup and then reaches your eyes. If you put the light on the same side of the cup as you, then you are looking at light which hits the cup, reflects or scatters off the glass and gold and then reaches your eyes.

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