Light Investigation

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We teach students that plants need Light, Air and Water to grow properly. But how does light affect plants? And why is light important for plant growth? By planting the same type of seed at the same time in three different amounts of light, students will be able to both observe and measure the affect of light on a plant’s ability to grow.


Students will be able to...

  • measure the affect of light on plants
  • observe firsthand how plants are able to “feed” themselves through photosynthesis. The plants given enough light look strong, healthy and well-nourished. The plants given no light are pale, weak and look unhealthy.


After all of the data has been collected, bring the students together for a discussion. Have the students use their data to explain how light affects plants. This is a great time to see if your students understand the concept of photosynthesis and why plants need light. Have your students write down their observations in their journal and check their journal pages to assess their understanding

Age group: 4th-6th

Venue/s: Greenhouse

Materials: Investigation table, pencils

Time: 45-60 min

Set up: Check with gardeners to see if they have set up the plants for the investigation in the green house.

The Lesson


Is light necessary for plants’ growth? Can plants grow without light? How much light do plants need? Today we are going to observe three groups of fava bean seeds. These seeds were all planted at the same time, in the same soil, and have been watered the same amount. The only difference between the three groups of seeds is the amount of light they have been given.  Some of the seeds have been given 12 hours of light per day, some have been given just the natural daylight of December and January (about 8 hours) and some have been given no light. Let’s examine the plants and see if there is a difference between the three groups.


  1. Observe the overall health of the plants. What color is the leaf and stem in each grouping of plants? If some parts are white, why? Compare the color differences or similarities between the three groups.
  2. Measure the height of the plants. Are some plants taller? Compare the heights of plants between the three groups. How about the leaves? Measure the width of a few leaves from each plant group and compare.
  3. Do the plants stand up straight? Or do they need support? Measure the thickness of the plant stem. Compare the thickness between the three groups.

Materials and Directions:

  • The gardeners have set up this observation in the greenhouse. The plant groups are clearly labeled. Have your students bring their journals and be ready to answer the following journal question: What is the relationship between plant growth (measured variable) and light (changed variable).
  • Begin by having students make visual observations of each plant groups’ overall appearance.
  • Next divide the students into three groups and give them rulers and/or measuring tape.
  • Each student group will be assigned to a plant group and will take measurements of the height of the plants, the width of the leaves and the thickness of the stems.
  • Bring the students back together to compare the data between the plant groups and discuss the similarities and differences.

Optional activity for February (when we start taking down the light investigation):


The fava bean is a legume, which means that it is also a “nitrogen fixer”. Nitrogen fixing plants are those that have the extra capability of taking nitrogen gas from our atmosphere and “fixing” it into nodules of nitrogen on their roots.  These plants are able to do this with the help of a common bacterium known as Rhyzobium. This bacteria and the legume plant work together to “fix the nitrogen” into a form that is easily accessible to other plants. Examples of nitrogen fixing plants are legumes such as peas and beans. (For more information visit this link:


Gently pull one of the fava bean plants from the soil (one from each group) and check the roots for small white nodules known as ‘nitrogen nodules’.  Count how many nodules are on the roots of the fava bean from the group with 12 hours of light, natural light and no light. Does one group have more than another? Why?