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# Shady: How Does Distance From a Light Source Affect the Amount of Light Received by a Plant?

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Source:
Author: Janice VanCleave

### Problem

How does distance from a light source affect the amount of light received by a plant?

### Materials

• scissors
• roll of wide paper with no designs, such as butcher paper
• small houseplant about 12 inches (30 cm) tall
• flashlight
• yardstick (meterstick)
• 3 crayons of different colors
• helper

### Procedure

1. Cut a square of paper that is about twice as wide as the widest part of the plant.
2. Place the paper on the floor and set the plant in the center of the paper.
3. Darken the room and hold the flashlight just above the plant.
4. Ask a helper to use one of the crayons to outline the plant's shadow on the paper.
5. Move the flashlight to about 1 foot (3 m) above the plant.
6. Ask your helper to outline the shadow on the paper with a crayon of a different color.
7. Move the flashlight to about 2 feet (6 m) above the plant.
8. Again, have your helper outline the shadow on the paper with the remaining crayon.

### Results

The shadow is smaller when the light is held farther above the plant.

### Why?

A shadow is the dark area cast upon a surface by an object blocking light. The plant blocks the light coming from the flashlight. The size of the plant's shadow indicates how much light the plant blocks, and thus indicates how much light reaches the plant. A plant closer to a light casts a larger shadow. This plant receives more light than a plant that is farther away from the light and that casts a smaller shadow.

Light is very important to many plant processes, especially to photosynthesis. During photosynthesis, plants use light energy absorbed by chlorophyll in the chloroplasts to change water and carbon dioxide into food for the plants. Without enough light, plants cannot photosynthesize enough and will die.

### Let's Explore

Does the shape of a plant affect the amount of light it receives? Repeat steps 1 through 4 of the experiment, using plants of different shapes, such as tall, slender cactus and an African violet with flat, spreading leaves.

### Show Time!

1. How does the size of a leaf affect the amount of light it receives? Ask an adult for permission to collect leaves of different sizes from indoor and outdoor plants. Place a sheet of graph paper on an outdoor surface, such as the ground or a table. Graph paper with large squares—1 inch (1.25 cm)—works best. A photocopier can be used to enlarge the squares on the paper, or use a ruler to mark a piece of typing paper into 1-inch (1.25-cm) squares.
2. Hold a leaf by its stem about 4 inches (10 cm) above the paper. Ask a helper to outline the shadow cast on the paper. Repeat this procedure for each leaf.

From your results, determine which of the leaves collect the most light. Prepare and display a chart showing the leaves in order from least to most light received. Take a photograph of the actual leaves, arranged in order of the amount of light received, and use the photo on the chart.

NOTE: You can perform this experiment indoors in a darkened room by holding the leaf about 4 inches (10 cm) above the paper and a flashlight about 18 inches (45 cm) above the leaf

3. Estimate the area of the leaf shadows drawn on graph paper in the two previous experiments by counting the number of squares covered by each shadow. Count each square that is completely covered, then estimate the squares that are not completely covered by the following method: If half or more of a square is covered, count it as one square; if less than half the square is covered, do not count the square. For example, for the leaf shadow shown in the diagram, a check mark is placed in each square in which half or more of the square is covered. An X is placed in any square of which less than half is covered. To estimate the area, you count only the check marks. Thus, the area of the leaf shadow in the diagram equals 14 squares.