In a plant photosynthesis experiment, which variable is deliberately changed?

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The main idea is identifying the factor the experimenter changes on purpose to test its effect. In a photosynthesis study, you deliberately vary light exposure because light energy drives the light-dependent reactions and directly influences how fast photosynthesis can proceed. By changing the amount of light, you can observe how the rate responds while keeping other conditions the same. The other factors—soil pH, the type of plant, and temperature—are held constant to ensure that any observed changes come from the light level, not from those variables. Soil pH can affect nutrient availability and enzyme activity, the plant type would introduce many genetic differences, and temperature also affects reaction rates. So the deliberate variable is the amount of light.