The rate of photosynthesis depends on the distance to the light source. In this interactive, the photosynthesis rate is measured as a color change of the pH indicator Bromothymol blue (BTB). Photosynthesis consumes carbon dioxide, which increases the pH. This can be visualized by adding the pH indicator BTB. BTB is blue at a basic pH over 8, yellow at an acidic pH below 6, and shades of green in between. Instructions: Drag the slider to see how the distance from the light source affects photosynthesis performed by the water plant Elodea. Select “Show oxygen” to see how much oxygen gas is produced by the plant and released as bubbles.