TBR CBT#1 Q16

[brood910]

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I dont understand the logic behind the answer to this question (attached the pic).

If only red light is shone on the material, wont it absorb it and reflect green light?
Why is it that it appears to be colorless? And since when gray is "colorless"?

 

[inasensegone]

There is no green light for the object to reflect.

Under table 2, it says that material B reflects Greenish-Blue. This means that it absorbs everything but green-blue.

According to table 1, green-blue corresponds to approximately 565-420 nm (use emitted color)

The light shone on the object in the question, is red light, which according to table 1 is approximately 650-750 nm (use emitted color)

Put all this together, and the 650-750 nm shone on the object is outside of the range (565-420 nm) that the object does not absorb (it reflects). Essentially this means that the object absorbs everything inside this range (650-750 nm). Also, keep in mind that the object only appears blue-green in white light which means that it normally includes the 565-420 nm, which is not present in the red light light situation.

You could probably also simply use the 2nd half of table 1. A reflected color of blue-green would correspond to ~ 582-667 nm absorbed wavelength. Therefore if you shine a light on it in this range, it will be absorbed.

Perhaps you mixed up reflected and emitted color? Remember, emitted color is a color of light that you can see in complete darkness. Reflected color needs light shone on an object to produce a color. The reflected color is a combination of all the wavelengths that are not absorbed. In the example, there is no green light for the material to reflect if only red light is shone on it.

 

[brood910]

There is no green light for the object to reflect.

Under table 2, it says that material B reflects Greenish-Blue. This means that it absorbs everything but green-blue.

According to table 1, green-blue corresponds to approximately 565-420 nm (use emitted color)

The light shone on the object in the question, is red light, which according to table 1 is approximately 650-750 nm (use emitted color)

Put all this together, and the 650-750 nm shone on the object is outside of the range (565-420 nm) that the object does not absorb (it reflects). Essentially this means that the object absorbs everything inside this range (650-750 nm). Also, keep in mind that the object only appears blue-green in white light which means that it normally includes the 565-420 nm, which is not present in the red light light situation.

You could probably also simply use the 2nd half of table 1. A reflected color of blue-green would correspond to ~ 582-667 nm absorbed wavelength. Therefore if you shine a light on it in this range, it will be absorbed.

Perhaps you mixed up reflected and emitted color? Remember, emitted color is a color of light that you can see in complete darkness. Reflected color needs light shone on an object to produce a color. The reflected color is a combination of all the wavelengths that are not absorbed. In the example, there is no green light for the material to reflect if only red light is shone on it.

Thanks for the response. This question has nothing to do with the difference between emitted light + reflected light, so no, I didnt mix up those two.

I thought reflected colors correspond to particular colors that are absorbed.
Ex: Red - green, blue - orange, yellow - violet.

When the white light was shone on the material B, didnt it actually absorb only orange + red colors, not all colors?

I guess it makes sense why the answer is grey tho, since if only red light is shone, then nothing is reflected as the material can absorb the red light.

 

[inasensegone]

When the white light was shone on material B, the reflected color is a combination of all the wavelengths that it did not absorb (i.e. that it reflected). Therefore it did not absorb any blue-green but it absorbed everything else.

 

[brood910]

When the white light was shone on material B, the reflected color is a combination of all the wavelengths that it did not absorb (i.e. that it reflected). Therefore it did not absorb any blue-green but it absorbed everything else.

Well, but the answer key says that it absorbs visible light in the red-orange range. That means it doesnt absorb all colors.

Reflected colors always correspond to particular colors that are absorbed as far as I remember correctly as I memorized R-G, B-O, Y-V.

 

[inasensegone]

That just means that that it has a maximum intensity absorbance of red (because the complimentary green is reflected) and orange (compliment blue). It doesn't mean that the other colors aren't also being absorbed in white light, otherwise they would factor into the reflected color.

 

[brood910]

That just means that that it has a maximum intensity absorbance of red (because the complimentary green is reflected) and orange (compliment blue). It doesn't mean that the other colors aren't also being absorbed in white light, otherwise they would factor into the reflected color.

So, for R-G, B-O, Y-V,
if red color is reflected, it means, the intensity absorbance of green color is greater than others?

 

[inasensegone]

So, for R-G, B-O, Y-V,
if red color is reflected, it means, the intensity absorbance of green color is greater than others?

You know, to be honest, I'm not entirely sure. But I think you may have over-thought this question (which isn't a bad thing because you want a deeper understanding obviously and that is good for the MCAT)... but as long as you realize that the emitted color matches the wavelength that is absorbed by the compound and therefore it will appear grey, you are good to go.