The correct answer to this open question is the following.
Unfortunately, you did not provide the sketch or the photo that shows what Francis is doing. So we do not know what you are talking about.
However, trying to help you, we did some deep research and can comment on the following.
If the photo shows a plan under the light of a lamp, moving towards the lamp direction, then tropism is Francis most likely investigating is phototropism.
To better understand the concept, Tropism is the movement of a plant towards the direction of an influence exerted on it. This means that an external factor is introduced in order to make the plant grow through an external influence.
So when we use the term phototropism, this means that the plant responds to the direction of the source of light and grows in that direction. That is what Francis is using in his science experiment.