1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Do objects reflect light and emit radiation simultaneously?

  1. Jan 22, 2015 #1
    Light hits an object and gets reflected back to our eyes and we see its color. But it also emits Thermal Radiation, so technically it should be 'emitting' both of those light waves at the same time.
    When iron gets heated up we only see the red color being emitted because it got heated up. What happened to it's original grey color?
    I don't know why but I feel like there's something wrong about this question
     
  2. jcsd
  3. Jan 22, 2015 #2

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Light and thermal are both electromagnetic radiation ( EM)


    as the thermal radiation increases in intensity, it goes from radiation we cannot see with human eyes through red, orange, yellow to white


    That's just its cold - cool colour, as it heats up its colour changes

    Dave
     
  4. Jan 22, 2015 #3

    DrDu

    User Avatar
    Science Advisor

    Yes, an object reflects and emits at the same time.
    As far as the colour of hot objects is concerned, they emit the colours they also absorb best. Search for "blackbody radiation".
    Maybe the best known example are metal vapours e.g. traces of sodium (also its chloride) brought into a flame. It emits strong yellow light. Yet when a bright source of light is shone through the flame, there are darker lines in the spectrum at exactly the frequency where the flame alone emits light. These black lines are also visible in the spectrum of the sun and lead to the discovery of helium, which was discovered only afterwards on earth.
     
  5. Jan 22, 2015 #4
    This is another thing I've wondered about.
    The color temperature chart. If the color emitted by Thermal Radiation corresponds to the object's temperature, or in other words, it's energy, and the higher the frequency, the higher the energy, an thus the higher up the electromagnetic spectrum the color is. But the temperature chart isn't the same as the color spectrum. There's no green color in the temperature chart. Why is that.

    Tl;dr Why is there no green color in the temperature chart. It goes from red to white to blue/purple.
     
  6. Jan 22, 2015 #5

    DrDu

    User Avatar
    Science Advisor

    The colour of glowing objects usually cited refers to ideal black objects, i.e. objects which would absorb light of any frequency completely. The maximum of the emission spectrum shifts with temperature (see Stefan Boltzmann's law).
    Objects which are strongly coloured may show other colours, e.g. Erbium oxide glows green!

    Edit: As Erbiumoxide is quite a common compound in a nerd's household, I had to try this out, as I have never seen it. The effect is indeed spectacular, although the photo is not as impressive. The green colour can best be seen in thre reflections on the aluminium dish.Green is roughly the complementary colour of the pink of the oxide at normal temperatures.
     

    Attached Files:

    Last edited: Jan 22, 2015
  7. Jan 22, 2015 #6

    DrClaude

    User Avatar

    Staff: Mentor

    If you plot the blackbdy emission curve for a temperature where the peak emission is at the wavelength for green light, you will see that it covers the entire visible spectrum nearly uniformly. Such light therefore looks white, not green. This is the same reason why there are no green stars.
     
  8. Jan 22, 2015 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    This discussion is an example of the confusion between Colour, Wavelength and Spectrum. Your eye is not a spectrometer and will only analyse the spectrum of incident light with three sensors with very broad band analysis curves. It does the best it can. Evolution must have a part to play in the explanation of 'why' we see what we see. It can't have been a very strong evolutionary requirement to discriminate the 'colour' of very hot objects (stars). I would say that the main interest in colour temperature would be to recognise sunset, red clouds, very bright sunlight etc. and so those are the things we are aware of.
     
  9. Jan 22, 2015 #8
    Could you help me understand this better.
    1421971111838.jpg
    Here's what I got from it.
    At 3000k the wavelength of the radiation being emitted from an object peaks in the infrared part of the spectrum. We see the red color because at 3000k it has its highest intensity inside the visible part where the red color is, right?
    At 6000k it peaks inside the visible part of the spectrum, and it's where the color is blue. But here, at 6000k, the intensity of the light emitted is way greater than at 3000k, so we should see a brighter blue than the previous red, right?

    Can the absence of a green in the color temperature scale be explained using this picture?
     
  10. Jan 23, 2015 #9

    DrDu

    User Avatar
    Science Advisor

    DrClaude already explained you why. When the frequency with maximal intensity corresponds to green, the spectrum is so broad and flat (we are at a maximum!), that we have the impression of white.
     
  11. Jan 23, 2015 #10

    DrClaude

    User Avatar

    Staff: Mentor

    Here is the simulated spectrum of the star 40 Eridani A, which peaks at ~ 550 nm, which is in the green part of the visible spectrum. I haven't superposed the visible spectrum like you did, but if you look in the region 400 - 800 nm, you will see that there is plenty of light at all wavelengths. Add to that the question of human perception, which sophiecentaur pointed out, and you get that we see this as white.
     

    Attached Files:

  12. Jan 23, 2015 #11
    Oh, alright, I finally understand it.
    Thanks a lot for you all.
    These questions have been bugging me for a long time.
    Sorry if I've caused you any trouble.
     
  13. Jan 24, 2015 #12

    tech99

    User Avatar
    Gold Member

    In the case of sodium vapour, the colour is decided by the band gap energy rather than the flame temperature. In fact, I notice that the dark lines are visible from the area above the luminous part of the flame, coming from cool sodium vapour.
     
  14. Jan 24, 2015 #13

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    That's not quite it.
    White is a perception. It does not have to equate to uniform, or even nearly uniform, power across the visible spectrum.
    In effect, white is any combination of wavelengths which generates the same response ratios from our three types of cone as does the ambient light from the sun.
    The sun appears yellow (from earth's surface) because the bluer light is scattered in the atmosphere, making it yellow relative to ambient light. In space, the sun looks white.

    The eye adapts quite a lot. You might regard a wall as painted pure white, only to realise it's a bit yellow or a bit pink when a whiter piece of paper is placed against it.

    There's an important distinction between what happens at modest energies and what happens at arbitrarily high energies. (I'll only discuss atomic behaviour, but there are also effects at the molecular level.) I'm sure other posters on this thread know more about this than I do, but it does not seem to have been discussed.
    At modest energies the emission/absorption is associated with electrons jumping between bound states. Since these have defined energy levels, only specific wavelengths are absorbed/emitted. (There is some blurring from quantum and Doppler effects.) Reflection occurs when the incoming light fails to match available transitions.
    At higher energies, transitions can occur between bound and free states. Since there is no upper limit to the energy of a free state, a continuous spectrum arises.
    A sufficient mixture of materials can offer so many transition energies that the spectrum is more less continuous even at modest energies.
    Glowing red hot or white hot typically represents such continuous or near-continuous spectra. White hot would correspond to reaching temperatures similar to that of the sun's surface.
    As mentioned, some stars are hot enough to glow blue to the eye (except, not with enough intensity to stimulate blue cones to the naked eye). I think that needs a temperature of around 10,000K.

    Edit: "Transmission occurs when ..."
     
    Last edited: Feb 10, 2015
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook