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Using Mirrors in the Garden: Measuring Light Reflection

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Author: Tricia Edgar
Topics: Fourth Grade, Botany

What will happen when you place a mirror in the garden? In this experiment, you’ll use a light meter to measure if adding a mirror to the garden increases the amount of light that’s available for your plants.

Problem:

Does light in the garden increase when you add mirrors to redirect sunlight?

Materials

  • Two large, old mirrors
  • Tape measure
  • Light meter
  • Notebook and pencil

Procedure

  1. Create a hypothesis, your best guess about what is going to happen. Will adding a mirror increase the light levels in the garden? Will they stay the same?
  2. Now, head outside. Choose a location that is partially shady and would benefit from more light. For example, this might be under a deciduous tree that provides dappled shade with its leaves.
  3. Use your light meter to take a reading of the light in the area. Make sure that you hold it away from your body so that your body doesn’t cast a shadow over it.
  4. Measure out a 3’ x 3’ square and take several light meter readings 2 feet off the ground in the square.  Add the readings together and divide by the number of readings you took to get the average.
  5. Now, install two mirrors facing each other in the area you’ve just measured with the light meter. Try your readings again. Take several readings 2 feet off the ground in the middle of the mirrors. Add the readings together and divide by the number of readings you took to get the average. Make sure you don’t cast a shadow over the light meter!
  6. Did the light levels change when you added a mirror? Why or why not?

Results

Light levels in some locations will increase, but overall the light levels will remain about the same.

Why?

A mirror is a light recycler, which is what makes this experiment so tricky. When you add mirrors to a home or a garden, they reflect and recycle the light that enters that space. If you add many mirrors, they can bounce the light off each other, and with each bounce, a little bit of light is lost.

If you measure a specific spot that’s very dark, then place a mirror near it so that it reflects light into that spot, then the place will get brighter. You have made that one space get light when it didn’t get light before.

However, mirrors don’t make a sunny day sunnier. A mirror can’t increase the amount of light coming in, but it can bounce it around once the light gets there. If you measure the light levels in your space, you’ll likely find that they’re about the same on average. If you measure the light in a particular location, you might find that it’s gotten a lot brighter if you’ve pointed the mirror’s reflection to that place.

You can use mirrors in the garden to move the light around, allowing plants to grow in areas where they could not grow before. Unfortunately, you can’t use mirrors to increase the total light that enters your garden—but wouldn’t it be nice if all we needed to do to have a sunny day were add a few mirrors to a garden?

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