Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why colours are reflected

  1. Jun 23, 2010 #1
    How come certain objects reflect certain colours (wavelengths) but not others?
    What is the relationship between the wavelength and the object? I approached my physics teacher and he had no answer and i have been desprete to get and answer, i've checked all over the internet to find one and came out with nothing. All i got was reflection, refraction and defraction, but this still didnt really answer my question

    Will be great for an answer
  2. jcsd
  3. Jun 23, 2010 #2
  4. Jun 23, 2010 #3

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor

    If the emissivity is a function of wavelength (alternatively, the absorption or transmission), the material will reflect certain colors.

    Although we do not yet have a detailed microscopic description that allows us to predict the color a material will appear, we do know a few things: why gold and copper look different than silver, for example.
  5. Jun 23, 2010 #4
    Are you asking how you see an object as red? Or are you asking why does a red object absorb blue and green (and all other wavelengths), but not red?

    You see an object as being a certain color b/c it's absorbing all the colors that it isn't reflecting... Okay, that's obviously circular... but here's what i mean... There are additive colors which are in the form of "light" and there are subtractive colors, which are "objects" or pigments. Take the example of using red, green and blue lights in the proper ratio to give white (sort of white, let's not get into the issue about color reproduction with RGB, and it's not 1:1:1 in terms of optical power, it's only 1:1:1 if you account for how our eyes respond to different wavelengths, which units like lumen and candella take that response into account, so 100 lumens of 550nm appears as bright as 100 lumens of 650nm, but the 550nm is less optical power). Anyhow, if you aim "white" light at an object and it appears red, then that roughly means the object is absorbing blue and green (and everything else other than red) so all that's left to "leave" the object is red.

    It can get complicated though b/c reproducing colors isn't really as simple as, "all you need to make any color is RGB." but I'm not going to get into that.

    Is that what you are asking, why do we see a red object? Or, are you asking the harder question to answer, "How/Why does the red object absorb blue and green, but not red?" (anything i've ever read just says things like, "The phsyical properties of an object determine what colors are absorbed."

    Talking about lumens and optical power got me thinking once again that even with all the advancements in high output flourescents, LEDs, and even unconventional microwave-sulfur lamps, we're still very inefficient at creating light. Even the most efficient light sources (like LOW pressure sodium) are around 200 lm/watt, but yet 1 watt of luminous power is over 600 lumens at 555nm.
  6. Jun 23, 2010 #5


    User Avatar
    Gold Member

    Yes, Alex your question is ambiguous. There are myriad reasons why objects reflect different colours.

    For example a red ball is red because it contains red pigements. The pigments are molecules that preferentially absorb most wavelengths except in the red band. Centuries of experimentation has lead us to these few chemical pigments that are pure and stable.

    Other objects, such as soap bubbles and oil slicks, have physical properties (thin films) that have effects on colours - the light rays interfere with erach other and cancel out or are reinforced.
    Last edited: Jun 23, 2010
  7. Jun 23, 2010 #6


    User Avatar
    Gold Member

    It is important to point out that the red/green/blue distinction of light's components is completely human-centric.

    We think that red, green and blue are all you need to make the entire rainbow because we have red, green and blue receptors. If all three are stimulated, presto, we see white light.

    This is a very narrow view of visible light though.

    There is an inifinite range of colours making up light white.
  8. Jun 23, 2010 #7


    User Avatar
    Science Advisor

    Typically, if a particular wavelength is not being reflected, it means that there is an electron somewhere in the substance at the surface that can go to an excited state with the amount of energy equal to the energy of a photon with a given wavelength. But it can be a lot more complicated than that as well.
  9. Jun 23, 2010 #8
    Sorry i didn't make it clear enough, i find it hard to define what i mean :D, i know that objects absorb certain wavelengths and reflect other wavelengths and that's why we see colours.. i want to know the fundimental reason, exactly what are the criteria of an object that allows certain wavelengths to be absorbed and reflect certain wavelengths
    I hope this has made it a little more clear to understand
  10. Jun 24, 2010 #9


    User Avatar
    Gold Member

    Ah. Certain molecules have atomic bonds that will absorb certain wavelengths of photon.

    A molecule might have a bond between two atoms that has some "wiggle-room" to it - the bond might twist or vibrate easily if given a certain kick. That kick will have to match a certain quantum of energy, for example, it might only absorb a photon that's 550nm.
  11. Jun 26, 2010 #10
    yes like dave is saying it has to do with the bonds , like diamond will let all colors of light
    go through it but graphite will not and they are both made of carbon but there bonds are different , you could make it more complicated by looking at what happens when light hits glass , will it get reflected or go into the glass and get refracted , it turns out it also depends on the thickness of the glass , if you make it thicker you could makes less reflect , but you make it thicker and you could get more to reflect and it keeps going back and fourth when you make the glass thicker .
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook