The rate of photosynthesis depends on the intensity of the light source. In this interactive, the photosynthesis rate is measured as a color change of the pH indicator BTB. Photosynthesis consumes carbon dioxide, which increases the pH. This can be visualized by adding the pH indicator bromothymol blue, or BTB. BTB is blue at a basic pH over 8, yellow at an acidic pH below 6, and shades of green in between. At the green arrow, the maximal rate of photosynthesis is reached. Increasing the light intensity beyond this point does not increase the rate of photosynthesis anymore. Instructions: Drag the light intensity slider to see how it affects the pH and the rate of photosynthesis of the water plant Elodea in a BTB solution. By checking the box “Show oxygen”, you can see the oxygen being released during photosynthesis as bubbles in the tube.