Push light up and the rate climbs — until it flattens. When it plateaus, carbon dioxide has become the limiting factor: raise that and it climbs again. Whatever is lowest is the brake.
Now reason backwards — you're handed the result, find the cause
A pot plant left in a dark cupboard for two weeks comes out pale, thin and weak. Why?
Work back from the symptoms. No light means no photosynthesis, so the plant made no new glucose and slowly burned through its stored food by respiring — that's the thin, weak part. And the green pigment chlorophyll fades without light to keep it made — that's the pale part. One missing ingredient, light, explains every symptom.
A piece of pondweed releases far more oxygen bubbles at midday than at dusk. Why?
Work back from the bubbles. More oxygen bubbles means a faster rate of photosynthesis. The one thing that changed between midday and dusk is the light level — bright midday sun drives the reaction hard, so lots of oxygen comes off; dim dusk light lets it idle, so only a trickle. The bubbles are a live read-out of the rate.