Reflectors: Why are Plant Leaves Green?
Why are plant leaves green?
- Manila file folder
- Sheet of white office paper
- Paper clip
- Sheet of green construction paper or any bright green paper
- Open the file folder slightly and stand it on a table.
- Attach the white paper to one side of the standing folder with the paper clip. The white paper will be called the screen.
- Lay the flashlight alongside the standing folder. Rotate the flashlight so that the lamp end faces away from the screen at a 45-degree (45°) angle.
- Fold the green paper in half lengthwise (long end to long end) to make it easier to hold.
- Hold the green paper about 4 inches (10 cm) from the lamp end of the flashlight.
- Turn on the flashlight, then ask your helper to darken the room.
- Observe the color of the screen.
The screen looks green.
The screen looks green because of something called pigment. Pigments are substances that absorb, reflect, and transmit (pass through) visible light. Visible light is made up of colors that can be seen by the human eye, commonly referred to as rainbow colors: red, orange, yellow, green, blue, indigo, and violet. White light, such as sunlight or light from a flashlight, is made up of the seven colors of visible light. When white light meets matter (the substance of which any object is made), any number of the different colors of light may be reflected, transmitted, or absorbed.
When a pigment is illuminated, or brightened, with white light, the color you see is the color of light reflected and/or transmitted by the pigment. (The rest of the colors of light are absorbed.) You see green on the screen because the pigment in the colored paper reflects green light. Similarly, you see green when looking at a green leaf because a pigment in the leaf called chlorophyll reflects and transmits green light. Chlorophyll is a green pigment located in the chloroplasts of plants. Chlorophyll is necessary in the process by which plants produce food, called photosynthesis. During photosynthesis, plants use light energy trapped by chlorophyll to change water and a gas in the air called carbon dioxide into food.
When white light strikes a red flower, the flower absorbs all the colors in the white light except red. The flower appears red because only red light is reflected. Demonstrate this by repeating the experiment, using red paper. Science Fair Hint: Construct and use a diagram like the one shown as part of a project display.
The colors of light that a pigment absorbs and transmits can be determined by an instrument called a spectrophotometer. Make a simple spectrophotometer by laying a sheet of white paper on a table. The sheet of paper will be your screen. Prepare a liquid containing pigment from a leaf, such as a geranium leaf, by tearing the leaf into small pieces and placing them in a clear plastic cup. Add 3 tablespoons (45 ml) of rubbing alcohol to the cup. CAUTION: Keep the alcohol away from your nose and mouth. Stir the contents of the cup as often as possible for 1 hour, then remove the pieces of leaf.
Hold the cup about 4 inches (10 cm) above the screen, and the flashlight about 2 inches (5 cm) above the cup. Shine the light through the liquid in the cup and onto the screen. The color seen on the screen is the color transmitted by the pigment removed from the leaf. The colors absorbed are all the colors making up white light, minus the transmitted and any reflected colors. Take a color photograph or make a color diagram to represent the results. Find out how professional spectrophotometers work.
Check It Out!
There are three groups of pigment in plants: chlorophyll, carotenoids, and phycobilins. Find out more about these pigments. Which colors does each group absorb, reflect, and transmit?
Warning is hereby given that not all Project Ideas are appropriate for all individuals or in all circumstances. Implementation of any Science Project Idea should be undertaken only in appropriate settings and with appropriate parental or other supervision. Reading and following the safety precautions of all materials used in a project is the sole responsibility of each individual. For further information, consult your state’s handbook of Science Safety.