The light intensity decreases with the square of the distance, so if the lamp is moved from 18 cm to 36 cm, the intensity is reduced to one-fourth (1/4th).
The relationship between light intensity and distance is described by the inverse square law, which states that the intensity of light is inversely proportional to the square of the distance from the source. In this case, if a lamp is initially placed 18 cm away from a plant, and it is then moved to a distance of 36 cm, the new intensity of light reaching the plant would be reduced to one-fourth of the original intensity.
This is because doubling the distance results in the square of 2 (2^2), which is 4. Therefore, the light intensity diminishes as the distance from the light source increases, following the principles of the inverse square law.