Examination How Different Coloured Rays of Light Affect Photosynthesis

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Examination of How Different Colours of Light Affect
The Rate of Photosynthesis


Plants are an essential part of the food chain required by all living things to survive. The growing world population and subsequent alteration to the natural environment has placed increased pressure on available plant resources. This is particularly evident in the area of agriculture, where optimum crop productivity is important to meet the increased demand for food by the growing world population.

Plants are autotrophic, which means they make their own food, sustaining life without needing to consume other organisms1. They do this through the process of photosynthesis. This is where light energy from the sun (or other radiant source) is trapped by green plants and transformed into chemical energy.

All photosynthetic organisms contain chlorophyll, a green pigment found in plant chloroplasts. There are three types of chlorophyll pigment – a, b and carotenoids. Chlorophyll a is the pigment primarily responsible for the process of photosynthesis. It is green as a result of reflecting green light rays. Light energy is captured by chlorophyll and used to form ATP molecules, which are in turn used as an energy source by plants in a chemical reaction with combines carbon dioxide and water to produce glucose and oxygen.2 The equation which describes the process of photosynthesis is:

Carbon dioxide + water light energy ► glucose + water + oxygen

Light energy is essential for photosynthesis to occur and this is usually provided in the form of sunlight. All light, including sunlight, is a collection of wavelengths. The visible light spectrum contains wavelengths which vary in intensity from around 350nm (nanometres) to 750nm (Figure 1). Chlorophyll reflects green light (so leaves appear green) and absorb more blue and red light 4 .

Figure 1: Wavelengths on the Visible Light Spectrum
(Source Britannica.com)

The rate at which photosynthesis occurs is influenced by the wavelength of light. The shorter a light wavelength, the more energy it has and the higher the rate of photosynthesis. That is, light energy is absorbed most effectively at shorter wavelengths. So, red light has more energy than green light5.

Knowing optimum light conditions to maximise photosynthesis (and therefore plant growth rate) could be considered useful information, especially to those involved in agriculture.


Plants require photosynthesis to produce food energy. If lightwaves at higher intensity result in a higher rate of photosynthesis, then plants grown under red will have a faster growth rate than plants grown under other colours in the visible light spectrum.


An experiment was designed to investigate the effect of different coloured lights on the growth rate of bean plants. The plants were grown under blue, red, yellow and green lights in controlled conditions over a period of time. A control experiment was established by growing plants under identical conditions using unfiltered (white) light. The height of each plant was measured periodically and the results tabulated and analysed.

Independent variables affecting the experiment included:
Colour of light (wavelength)

Dependent variables affecting the experiment included:
Growth rate of plant

Controlled variables affecting the experiment included:
Starting height of plants
Size and type of container
Soil type and volume
Duration of light applied
How deep and far apart planted

Uncontrollable variables affecting the experiment included:

The group of plants growing under each colour light, plus the control group, were individually measured on a regular basis and the average plant height of each group was calculated. The rate of growth was analysed by calculating the change in growth of each group of plants under the different lights and...
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