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Light Penetration

Name of Corresponding Unit Plan: Aquatic Environments

Grade Level: 5-12

Common Core Standards:
RS9-10. 3. Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks, attending to special cases or exceptions defined in the text.

M.5.G.1. Graph points on the coordinate plane to solve real-world and
mathematical problems.

Content Areas: Science

Recommended Length/Duration: 45-60 minute period

Learning Goals: Students will understand how particulate in water will increase turbidity and limit the penetration of light through water.


  1. Discuss how the underwater environment has both vertical and horizontal visibility.  Vertical visibility is important for seeing objects on the bottom or for determining the depth to which photosynthesis can take place.  Horizontal visibility enables organisms, including scuba divers, to see the environment around them.
  2. Visibility is determined by the extent to which light can penetrate through the water.  Particulate in the water will reflect and disperse light, limiting its penetration.
  3. Divide students into lab groups and have them set up the experimental apparatus:  aquarium filled with clear water, flashlight or lamp mounted horizontally, light meter (a light meter app can be downloaded free on most smart phones).
  4. Explain that students will be gradually adding particulate to the water in their tank to increase the turbidity.  Have record a hypothesis as to what will happen to visibility.
  5. Have them take a baseline light measurement and record their reading on the worksheet data table.
  6. Have students add 250 ml of particulate to the aquarium and stir vigorously.  Take another light meter reading before any of the particulate has precipited.
  7. Continue to add particulate, 250 ml at a time, until the visibility has been reduced to nearly zero.
  8. Have students graph their results.
  9. Discuss whether or not their hypothesis was right and if there is any pattern to the light penetration data they recorded.  Guiding questions might include:
    • Was there a relationship between the amount of particulate and light penetration?
    • Was there a pattern to the reduction of light penetration?
    • Were you able to reduce visibility to zero?  If not, how much particulate would be needed?
    • What did the water sample look like as you added particulate?  How does that compare with natural water bodies you are familiar with?
    • How would high or low levels of light penetration affect the aquatic ecosystem?

Assessments: Informal assessment based on participation and effort.  Check worksheets for accuracy.

The final discussion question could be the topic for a formal essay evaluated with an assessment rubric.

Materials/Resources: Light Penetration Worksheets (pdf), Aquarium, flashlight or lamp, light meter

Special Considerations: This experiment will work best in a darkened room.

Students may need instruction on how to read the light meter that is available.  Different light meters may read in various units of measurement.

Aquariums may be replaced by a large jar.

The activity may be done as a demonstration experiment if only one aquarium or light meter is available.

Extensions: Students may want to measure water samples taken from different natural sources.

Students may want to try light sources of differing intensities to see if the pattern of penetration is consistent.