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B Difference between color / temperature?

  1. Aug 6, 2018 #1
    What is the difference between a red object at room temperature and a heated red object (Metal for example).

    They are both red except one is cooler then the other?

    Why aren't the red objects in my room not hot like the heating of a metal?
  2. jcsd
  3. Aug 6, 2018 #2


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    Red means some electromagnetic waves of a certain wavelength fall into your eyes. It does not allow to conclude, where those waves were created from. In case of a hot metal the source is the electrons of the heated metal atoms. In case of a red flower, it is the daylight minus all wavelengths which the flower absorbs, i.e. only red isn't absorbed but reflected. Now daylight comes from the sun, which is hot, too, only farther away. However, I suppose the creation process is a different one.
  4. Aug 7, 2018 #3


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    The spectra are different. For a hot 'black body' the spectrum covers the whole of the EM spectrum with a peak around the 'red' wavelengths and, for a pigment, the spectrum of reflected (white) light will be a band of wavelengths around the red wavelengths (depending on the quality of the pigment.
    Our eyes are very poor spectrometers and, under the right viewing conditions, those two 'red' colours could be completely indistinguishable. We could possibly distinguish between two test objects by the fact that we could feel the IR wavelengths against our face and conclude the hot object is in fact hot but the cold red object is not. But the eyes would probably not help in deciding which is which.
    If the EM from a red hot object were passed through a good red filter, the spectrum would be pretty much the same as when white light is passed through the filter because the black body spectrum is broad and doesn't change much over any chosen narrow band section.
  5. Aug 7, 2018 #4


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    I proved to myself a couple of years ago that there is green light present in a red/orange hot electric stove burner. (≈1500°F / 800°C)

    This image was taken through diffraction grating.

    I also captured the fuzzy spectrum of the neon "On" lamp on the front of my stove, which is a different kind of "color" altogether. (I think. :biggrin:)
  6. Aug 7, 2018 #5


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    Oh my.
    For several years I've been wanting to do a "spectral analysis" of a banana, to figure out if it's true that they aren't really yellow.
    Your question finally prompted me to do the experiment.
    Albeit, I used a lemon this morning, as, I kept eating my previous experimental subjects.....

    Anyways, here are the results of this morning's experiment:

    Experimental apparatus:

    Subject of experiment:

    Results of experiment:

    Conclusion: Lemons are white? :oldconfused:

  7. Aug 7, 2018 #6


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    Conclusion: Eyeballing intensity levels at different frequencies and is not a good way to quantitatively measure a spectrum. The logarithmic response of the eyes will tend to make every frequency appear to be illuminated with rough uniformity.

    Conclusion: Every experiment needs a control. If you look at the spectrum for an apple, or a piece of white paper compare it to one for a lemon and fail to find a difference then you have a real problem with your methodology. If you do find a difference, now you're getting a meaningful result. Side by side recommended -- eyes are designed to detect differences in illumination, edges.
  8. Aug 7, 2018 #7


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    Run the eye dropper sample tool on your Photo processing program and notice the RGB values on the image. There may be a Luminance option button - or convert to monochrome. That will show you a difference when compared to a white card (in shadow to avoid 255 255 255 saturation)
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