When the rate of photosynthesis is limited by light, the rate is directly proportional to the light intensity. If the light intensity doubles, the rate doubles. The light intensity itself depends on how far the plant is from the lamp, and it falls as the distance increases.
The link is the inverse square law: light intensity is proportional to 1/d2, where d is the distance from the light source. If you double the distance, the intensity falls to one quarter. If you triple the distance, it falls to one ninth. If you halve the distance, the intensity becomes four times bigger. So the rate of photosynthesis is also proportional to 1/d2, as long as light is the limiting factor.
Worked example: a plant 10 cm from a lamp makes 36 bubbles per minute. It is moved to 30 cm. The distance is three times bigger, so the intensity is 1/32 = 1/9 of the original. The new rate is 36 ÷ 9 = 4 bubbles per minute. A graph of rate against 1/d2 gives a straight line through the origin.