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Intresting question

  1. Oct 25, 2011 #1
    If we see the green grass in night with red light what colour would we observe ?
  2. jcsd
  3. Oct 25, 2011 #2


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    Well, the majority of the pigments of the grass reflect green light (which is why it looks green under white light). But there will be some pigments which reflect other wavelengths as well. So if we shone purely red light on some grass, then the pigments which do reflect red light will reflect it. So the reflected light would just be red, and the grass would look red (but much fainter, since less light is reflected).

    If the light we shone on the grass was mostly red, but some other wavelengths as well, then the grass will have a chance to reflect those other wavelengths. The grass reflects green the best, but there is more red light. So there will be mostly red and green light reflected.
  4. Oct 28, 2011 #3
    U said that it reflects red, but grass is green it needs green colour to reflect and it has not green colour so it is not possible and as the colour of grass is green so it is also impossible for it to reflect red
  5. Oct 28, 2011 #4
    According to my opinion it seems black because it is in the need of green which is not avilable and when nothing to reflect it seems black
  6. Oct 28, 2011 #5
    It will depend on how red is the red light. "Green" and "red" correspond to some wavelength ranges rather than single values. Chlorophyll in the grass has strong absorption peaks in the blue and red regions and strong reflection in the region between - green and yellow.
    The minimum of reflection is around 670-680 nm. Light with shorter wavelengths, like 620 nm, is still red but is reflected more. Anyway, the minimum reflection does not mean zero reflection.
    Some red light will be reflected. What you see it's another matter. It may depend a lot on the contrast with the background and the absolute intensity of the red light shined on the grass.
  7. Oct 28, 2011 #6
    Green grass reflects green light (and some light in other areas of the spectrum). Theoretically, if the surface of the grass blades was non-reflecting (no shiny surfaces), it should appear as really dark when illuminated by a normal red light in darkness.
    If you used a red laser though, the grass would reflect a lot of it back to you and so it should appear red (since no other colors are present in the laser light).
  8. Oct 28, 2011 #7


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    @NIKHEL: green grass is not ideally green. It is not even close to such, as green reflecting etalons could be. So it reflects also some red light. If you lit it with purely red light it will still reflect some - just less than if lit by green light. And the reflected light will still be purely red.

    Take you (nearly perfect) red light source: a toy red laser pointer and check it yourself: you may see the red spot on grass on leaves.
  9. Oct 29, 2011 #8


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    I think you should investigate this, Nikhel. There are tail lights available for bicycles, comprising blinking red LEDs powered by 2 AA cells. Some have a switch allowing a steady bright light in place of blinking. In any case, the observation won't take you very long so the likelihood of a blinking-light-induced epileptic turn is minimal. I expect you will find the grass appears a very dark grey. Though some grass is light green, some very dark, and this will in turn colour your observations. :smile: You can probably borrow a light from a cyclist. Some clip it to the back of their helmet, so it's not a fixture.
  10. Oct 29, 2011 #9
    Grass is made up of elements formed into a biological sytem in wich Chlorophyll is a major part (but not all) to convert the light to energy. to energy. in most plants this is what gives it the green pigment you refer to as green grass. As others have stated the reflection of the light of a plant may not only be green.

    Try your experiment and see if your hypothesis holds. Such is the method use to confirm or discount such hypothesis as you statement "According to my opinion it seems black because it is in the need of green which is not avilable and when nothing to reflect it seems black'
  11. Oct 30, 2011 #10


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    Glad you are taking in interest in this sort of phenomenon, which many people don't even think about.

    Your way of stating things does not use the usual terms that Scientists use. This makes it hard to know exactly what you are saying. I am not just being picky when I say that it could help if you try to use the generally accepted terms if you want to be understood properly. You can improve in this by reading sources like Wikipedia and text books and copying the style and words.

    The word "need', for instance, implies "wanting", which is something that inanimate objects just don't do.

    Describing things as a chain of events or causal relationships can help. So you might say:
    The spectrum of a red light can be a mixture of wavelengths.
    An apparently green surface may reflect a range of wavelengths.
    If part of the spectrum of the 'red' light source is reflected by the 'green' surface then the surface will appear dark red. If the red and green are 'pure' then the surface will appear black

    Other posts have implied this, of course, but you don't seem to have taken it on board yet. That's why I have 'spelled it out'.
  12. Oct 31, 2011 #11


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    Staff: Mentor

    I tried just what I suggested for you. The colour of dark green grass blades under red LED light was what I would describe as a pinkish lilac.

    I expect a sheet of grey coloured paper would appear exactly the same. :smile:
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