How does light intensity affect plant growth?
This is a rather strange question and it can not be answered in a short comment that is normally done in this system! Such questions are often from young students looking for answers that have come from their teachers, and we are not in the business of doing your studying! There are dozens of books written on the influence of light quality and intensisty on the physiology and growth of plants. These are in libraries and many are also available online!
Each plant group reacts differently and has different physiology to deal with light intensity. Some plants do well in low light intensity and would be "burned" with high intensity light while other plants can only do well in full sunlight for long periods of time. If these are put into low light, the slow their growth and eventually die.
With that said, plants generally use light to perform the process of photosynthesis which uses the light energy to combine CO2 with water to make sugars and 02. The higher the light intensity, the more rapid this process can proceed. Thus, most plants grow faster in higher light intensities. Many plants actually will appear lighter green in high light intensity because the chlorophyll in the cells is sufficient to make all the sugar that the plant needs. These same plants may appear darker green in low light intensity because they need more chlorophyll to make the same amount of sugar in low light. Also, in general, in high light intensities, leaves will be smaller and the internodal distance (distance between where leaves and branches emerge) will be shortened.
By searching the internet using "plant growth light intensity" as the search words, I come up with hundreds of web sites that more thoroughly discuss this topic. I would recommend that you do this kind of search! As and example, look at: