The practical aspect of the physics behind our practical became somewhat irrelevant upon our discovery of the negligible effect of photosynthesis on the rate of fermentation of yeast. Despite this, the theoretical aspect of the physics behind the practical still stands itself in good stead.
The theory behind the experiment was the effect of differing wavelengths producing different energies. The formula E = hf, where h = Planck's Constant (6.6 x 10^-32 Js) proves that there is a relationship between the varying frequencies of light and the Energy produced. This relationship can be expanded to the formula v = fλ, whereby given the constant velocity of the speed of light (3x10^8 ms^-1), it can be seen of the inverse proportionality of frequency and wavelength. The effect this has on the experiment is the inverse proportionality of wavelength to energy. As the wavelength of the light increases, the energy decreases and whilst in practice this has no effect on the rate of reaction, the theory behind the action, despite not being observable is still theoretically sound.
The wavelength was measured by way of a wavelength spectroscope that allowed us to approximate to an appropriate amount, the wavelength of light passing through in m of each piece of cellophane.