SUPPLIES and PEDAGOGY

The "Photo" Side of Photosynthesis

In the A.P. Biology Lab Manual's chapter on Photosynthesis and Its Pigments, there is a classic example of the confusion of science and technology. A goodly percentage of the exercise is devoted to having the students determine which parts of the spectrum are absorbed by chloroplasts. That is important, but the way the students are introduced to it is literally "black box" methodology. For the first time in their life they have placed in front of them a spectrophotometer which they will use in a manner that the device was not designed for! Not only do the students have no reference between wavelength and particular colors, but they are expected to ascertain that by using a "black box" into which they cannot see what is happening. (And, as said, the Spectronic-20 and similar devices are not designed for efficient spectral work. For that double beamed devices were invented, but even those would be "black boxes" to beginning students.) It might even be interesting to give a quiz to teachers to see if they know which colors correspond to various wavelengths.

So let us arrange our pedagogy towards the end of getting basis science across to the students' minds!

SPECTROSCOPY
1. PEDAGOGY. If you want to know which wave-lengths are absorbed, THE device to use is a spectroscope, which is MUCH less expensive AND more straightforward than a spectrophotometer. Even the $5 refraction grating spectroscope of Nasco (Cat. No. SB11147M) works well. Were a student to see first the spectrum of the sun, and then interpose a green leaf between the sun and spectroscope and see that most of the red and blue portions of the rainbow have disappeared, well-, doesn't that plainly show that leaves absorb the red and blue ends of the spectrum and allow the green to pass through. Isn't that why leaves are green? Doesn't that tell you that green chloroplasts don't use the green portion of the spectrum? And how long does that take, after the kids get used to playing with the spectroscopes to look at the pretty rainbow colors? Seconds, and it is fully understood. No "black box" mysteries!
2. SUPPLIES.
  1. One of the following from, for example, Nasco:
    1. SB11147M Refraction grating spectroscope: $5 (or 15 of them for $34)
    2. SB26428M Prism spectroscope: $37
    3. SB21483M Refraction grating spectroscope: $91
  2. You may need to focus your spectroscope
    1. overhead fluorescent lights (why not incandescent lights?)
    2. a piece of NaCl sitting atop the grid on a burner (emits the very bright sodium lines).
  3. Various leaves, clear colored plastics, flower petals.
The RAINBOW
1. PEDAGOGY - 1. The flip side to the above is equally intellectually "transparent." Start with a rainbow, converge it all by projection into one WHITE spot (that's pretty amazing in itself!). And then to continue what seems almost like a magic show, there are three things that can be done with this system:
  1. Look at the white spot with a spectroscope, and the rainbow reappears - all the colors are there.
  2. Mask parts of the rainbow, and the spot changes colors, and the spectroscope reveals just which parts of the rainbow were masked.
  3. If both red and blue sections of the rainbow are blocked, the spot becomes green. That is rather obvious since there is only green left to be projected. There should thus be no mystery remaining in why leaves are green! And all this took how long? Maybe a minute!
2. PEDAGOGY - II. To make the above even more "transparent" to understanding, invited the students to stand around the projector so that they see everything you do, and that you are not doing any slight of hand magic.
3. SUPPLIES.
  1. A transparency projector and screen
  2. A transparency of a large rainbow (print this out on a transparency using a color printer)
  3. Picture matting (opaque paperboard) to be used for masking around the edges of the rainbow
  4. Other masking for blocking out portions of the rainbow
  5. A large (8 x 11 inch) magnifying lens or "sheet" obtainable in stationery and book stores ($5). This is to be used for converging the projected rainbow to a single spot.
  6. Ring stand and clamps to hold the magnifying sheet.
  7. Spectroscopes
Further PROJECTED FUN
1. PEDAGOGY. Project the primary colors onto the screen. What does your spectroscope see?
  1. A transparency of the three primary colors - red, yellow and blue.
  2. Maskings, etc., used in the preceeding section.
  3. Spectroscope
2. Do you see only red, yellow and blue in the spectroscopic view of the white spot? Or do you see the whole rainbow?