This was a tough challenge. I think you did an acceptable job of relating the theme of "Pi" by doing your measure of the radius of the lemon slice...
The composition on this photo is also good... positioning the lemons on the left of the frame and using the ruler as a leading line works very well.
The exposure on this image is where the problem lies. It appears that it's simply overexposed. I don't know much about the camera you are using so i couldn't suggest exposure alterations unless it has fully manual control. What looks like over exposure could also be a difference in your monitor and mine. If your monitor is way out of calibration, you may have increased the brightness on this image to a level that is too high on a calibrated monitor.
I'm not crazy about the composition, and the background seems kind of an odd color, and is very drab. The ruler blends into the background and doesn't really seem to be part of the subject.
It is round and showing measuring pi. The focus appears more on the ruler area, that made the lemon blurry. The backround has some distracting qualities and maybe using a solid instead of clear ruler may have given more impact.
This is a super idea with the ruler and the lemon slice. The title confuses me, though. I also feel that the lemon should be in focus and that the ruler should somehow stand out more to communicate your message here.