Example. Plotting solar radiation intensity (y-axis) as a function of wavelength. Wavelength will be the primary x-axis and the energy of the photon for that particular wavelength will be the secondary x-axis. The secondary axis will be inverted for this particular graph. as wavelength and energy are inversely proportional. This is useful cause for silicon bandgap energy of 1.1eV i can see from the graph how much radiation will it be able to absorb.