By Amelia Rowan, 12SDa
To discover whether the colour of the light source has any effect on the rate of photosynthesis of a sample of algae.
It was predicted that the colour of the light source would have a very slight effect on the rate of photosynthesis. It was further predicted that when the sample was placed under the green light, the rate of photosynthesis would be the lowest. This hypothesis was arrived at by taking into account the theory of light absorption – that pigments selectively absorb and reflect different spectra of light. Therefore, it was predicted that the rate of photosynthesis would be slowest for the green light as the chloroplasts that absorb light are green and would thus reflect green light.
Independent variable: The colour of light is the independent variable
Dependent variable: The rate of photosynthesis is the dependent variable
Variable How it will be controlled Why it will be controlled
Temperature The temperature will be controlled by carrying out the experiment in a dark room, which has a steady temperature due to being cut off from the outside environment. As temperature is one of the factors that affects the rate of photosynthesis, it is essential that the temperature is kept stable so the changes that we measure in the rate of photosynthesis are due to the independent variable, where the colour of light is changed.
Light source The light source used will always be 150W lamp This way the brightness of the light will be kept the same, but also because lights of a standard 40W or 60W produce a spectrum of light that is too limited for full activity.
Other light than controlled light source Any other light (other than the coloured light source) will be controlled by carrying out the experiment in the dark room overnight. As the purpose of the experiment is to determine whether the colour of a single