High School Experiments on the Effects of Different Lights on Chlorophyll

Written by john brennan
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High School Experiments on the Effects of Different Lights on Chlorophyll
Chlorophyll is the pigment that gives plants their green colour. (Liquidlibrary/liquidlibrary/Getty Images)

Chlorophyll is one of the main photosynthetic pigments in plants, meaning it plays a key role in the process that transforms light to chemical energy. But chlorophyll isn't equally good at absorbing all wavelengths of light. It reflects green light, for instance, which is why leaves are green in colour. You can design some simple experiments to investigate how these differences affect plant growth.

Colours of Light

Different wavelengths of light correspond to what we perceive as different colours. Light with a wavelength of 700 nanometres, for example, is red, while light with a wavelength of 400 nanometres is violet. To determine which wavelengths of light are absorbed by chlorophyll, you could test the effect of different wavelengths on plant growth. If light at a given wavelength is poorly absorbed by chlorophyll, the plant will not be able to obtain the energy it needs and it will not grow.

Experimental Design

The simplest way to design your experiment would be to place a soil tray beneath a cardboard box and insert cardboard partitions that divide the soil tray into five or six different sectors. Next, cut openings in the top of the cardboard box. There should be one opening per section. Finally, you can cover each opening with plastic or coloured glass. When you shine white light onto the box, the coloured plastic or glass will filter out all light except a given colour. By growing plants from seed in each section of the soil tray, you can test the effects of different colours on plant growth. It's important to remember that your experiment should also include a control -- seeds grown under white light -- so you have a standard of comparison for your results.

Types of Light

You could also run an experiment to test the effects of different light intensities on chlorophyll. Do plants grow better as the light intensity increases? You could set the experiment up in a similar way, but this time cover the opening to each box with tissue paper to filter out some of the light reaching the plants without subtracting particular colours. Adding additional sheets of tissue paper will reduce the amount of light available to each plant further still. Alternatively, you could try to determine whether the light source makes any difference. Try growing some plants under fluorescent light and others under natural sunlight (i.e. outdoors). You'll need to make sure the conditions for both groups of plants (temperature, moisture, air, time exposed to light) are the same in every other way so that only the source of light is different.

Chlorophyll Extraction

A fourth kind of experiment you could perform involves extracting chlorophyll from the plants' leaves. Start by tearing up spinach leaves and adding them to a mortar together with a little acetone; next, grind the leaves with a pestle until the acetone turns green. Transfer the mixture to a test tube, add 2ml of water and 2ml of hexane, then cap the tube and shake it gently. Open it away from yourself and others to vent it, then recap it and shake it some more. The mixture should separate into two layers, one of which is green. Using a pipette, draw off the other layer; the remaining mixture will contain the chlorophyll.

When extracted from the cells, chlorophyll will fluoresce under ultraviolet light. Try holding the chlorophyll under a black light or different kinds of lights to determine what colours or effects you see. Unfortunately, you will need access to some lab chemicals and equipment to carry out this procedure, but you can always ask your chemistry or science teacher whether she can help you find the necessary materials.

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