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INTRODUCTORY TEXT TO THE LESSON When the students were walking by the river the abundance of trees along the pathway attracted their attention. Ireland was once practically covered in trees and civilisation meant that many trees were taken down for use as fuels or construction materials. However, there are still many areas with significant tree canopy.
As we visited the area in different seasons the amount, colour and aspect of the leaves on the trees changed.
We were able to discuss the function of the trees and in particular the photosynthetic function of the leaves.
Both chloroplasts and the leaves themselves are different shades of green due to the pigments they contain. A pigment is a compound that absorbs a particular wavelength of visible light. Chlorophyll is a green pigment contained in membranes of chloroplasts that is used in photosynthesis by plants.
Photosynthesis requires light, but plants don’t use all the colors that make up white light. Using a spectroscope you can explore the absorption and transmission of white light through leaf material.
Some of the third level students were able to show how you could make a spectroscope from readily available materials.
A visible light spectroscope using a pringles potato chip can, old CD rom disc, piece of baking tray foil, tape, and a piece of matt black paper can be made in about 15 minutes.
This is demonstrated in the E-lesson.
Build the spectroscope:
Use the hole punch to make a hole in the center of one of the stickers. Position the diffraction grating on the adhesive side of the sticker so it covers the hole you just made, then press it into place.
Center the sticky side of the sticker with the diffraction grating over one end of the tube. Press and crimp the sides of the sticker around the end of the tube to secure it in place. This creates your eyehole.
Use scissors to cut the second circular sticker in half, so that you have two semicircles. Position the two half circles, adhesive side down, over the end of the cardboard tube opposite the diffraction grating so there is a very small gap between them, creating a narrow slit. Don’t press the edges of the stickers down yet.
Check your spectroscope and reposition the stickers as needed to align the diffraction grating and slit. (The diffraction grating has many small parallel scratches in the surface. The spectroscope works best if the slit created by the two half circles is parallel to the scratches in the grating. Although the scratches are too small to see with your naked eye, check the alignment by pointing the slit end of the spectroscope at the sky, clouds, or a brightly lit surface (NEVER directly at the sun) and looking through the hole with the diffraction grating. Look for the spectrum off to the side of where you see the slit. Adjust the position of the stickers until the spectrum looks like bands of color, not stars.
When you are satisfied with the look of the spectrum, crimp the edges of the sticker halves over the end of the tube to secure them in place.
Prepare your leaf filters:
Cut three pieces of each type of leaf, each large enough to cover the slit at the end of your tube completely.
Cut a piece of white paper to the same approximate size as the leaf pieces.
Point the slit end of the spectroscope at the sky (NEVER directly at the sun), at clouds, or at a light-colored surface in the sun (like a sidewalk or the wall of a building). Bring your eye right up to the other end of the tube next to the hole with the diffraction grating on it. What do you notice? What colors do you see?
Place the piece of white paper directly over the slit on the spectroscope as you point it toward a bright light source (not directly at the sun). Observe.
Remove the white paper and replace it with a layer of leaf material, making sure the leaf completely covers the slit. You may need to tape the leaf over the end of the spectroscope to prevent light from leaking around its edges. Point the slit end toward a bright light source (not directly at the sun). Does the spectrum you see look different now? Add another piece of the same type of leaf to the one already covering the slit of the spectroscope. What do you notice? Add a third leaf of the same type. How does what you see now compare with the full spectrum?
