Intensity of light after it passes through a convex lens When a parallel beam of light falls on a convex lens and get converge to its focus, does the intensity of light change?
 A: Yes, the intensity changes, because intensity is just energy per area per second.  You might be thinking of the related concept of "specific intensity", which is also called "brightness", which is the energy per second per area per incident solid angle (and can also be per frequency bin, but that's not of importance here).  The key part is the "per incident solid angle", because it means that the specific intensity of a light source, say our Sun, does not change when you get farther away.  Instead, the Sun just looks smaller, so occupies a smaller incident solid angle, and that's why it looks less bright though has the same formal "brightness" in each tiny incident solid angle bin.  The specific intensity does not change in a lens, what happens is the apparent size of the source is distorted.  So using a lens to burn a spot on paper is like making the Sun look larger at that spot, while keeping the same specific intensity.  But that means a larger intensity, because intensity is integrated over incident solid angle.
