How could I measure the rate of photosynthesis in a scientific experiment that'll test the effect of light intensity on the rate of photosynthesis with an Elodea plant?

There are a few key methods to calculate the rate of photosynthesis. These include:

1) Measuring the uptake of CO2

2) Measuring the production of O2

3) Measuring the production of carbohydrates

4) Measuring the increase in dry mass

As the equation for respiration is almost the reverse of the one for photosynthesis, you will need to think whether these methods measure photosynthesis alone or whether they are measuring the balance between photosynthesis and respiration.