Scientists call the intensity of light falling on an object the irradiance.
It depends upon two factors - the power of the light source and the area of
the beam of light falling on the object. The power of the bulb is measured as
the energy transferred per second in Watts. The area of the beam of light falling
on the object depends on how far away it is from the lamp.
The graph shows how light intensity decreases as the lamp is
moved away from the plant. You can see that the graph is not a straight line.
As the distance of the lamp from the plant increases the light intensity decreases
as an exponential curve. A straight line would show that light intensity
was proportional to 1/distance, and this is clearly not the case.
Light intensity
is however, proportional to 1/(distance)2, which is not really surprising since
it depends upon the area of the beam of light falling on the plant.
A graph of rate of photosynthesis
against 1/d2 should give a straight line for those range of light intensities
where light is the limiting factor to the rate of photosynthesis. Once light
reaches its optimum intensity, there will be no further rise in the rate of
photosynthesis.
What is the optimum distance
from the lamp for the Elodea in your investigations?
Light intensity and Elodea