Spectral Properties of Photosynthesis

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RED
Long λ
(low energy) 		BLUE
Short λ
(high energy)

  1. SPECTROSCOPY

    1. PEDAGOGY. For more than a century astonomers have been using spectroscopes to measure various properties of the sun and stars. The spectral properties are very revealing of the chemistry of stars. Applying the same technology to observations of such things as leaves or flower petals can be similarly educational. Let us now ask a pair of questions: Why are leaves green, and what does that say about the biochemistry behind photosynthesis?

    2. MATERIALS AND METHODS
      1. Spectroscope
      2. You may need to focus your spectroscope
        1. Focus on the bright mercury emission lines as seen from overhead fluorescent lights (why not incandescent lights?) Record the colors of those lines. Shown here is the expensive model.

        2. Focus on the very bright sodium lines from a piece of NaCl sitting atop the grid on a burner. Record the colors of those lines.

      3. Go out and collect various leaves, clear colored plastics, and flower petals.
      4. Look at them as they are placed between the objective of the spectroscope and the sun or other very bright incandescent light source.
      5. Do any of the portions of the spectrum (rainbow) dim in relation to other portions?
      6. Record your observations, and then move to the next part.

  2. The RAINBOW

    1. PEDAGOGY. Watch your teacher project a transparency of a rainbow onto the screen in front of the room. (For an image of a spectral rainbow that can be printed onto a projection transparency, click this button.) Then, by properly placing a large magnifying lens between the projectors mirror and the screen, the image will be converged into a spot on the screen. What color is it? Pretty amazing, isn't it? Now on with the magic show. First, gather yourselves around the projector so you can closely watch what the teacher is doing.
      1. Look at the spot on the screen with a spectroscope. What do you see? Record that observation.
      2. Mask parts of the rainbow, and the spot changes colors. Does the spectroscope reveal just which parts of the rainbow were masked.
      3. If both red and blue sections of the rainbow are blocked, what color does the spot become? Is this anything like what happened earlier when you looked at leaves through the spectroscope? Record your observations.

  3. Further PROJECTED FUN

    1. PEDAGOGY. Project a transparency of the converged primary colors onto the screen. What does your spectroscope see? Do you see only the three colors? Or do you see the whole rainbow? Of course, record your observations.

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