Aconvex lens is placed on a flat glass plate and illuminated from above with monochromatic red light. when viewed from above, concentric bands of red and dark are observed. what does one observe at the exact center of the lens where the lens and the glass plate are in direct contact?

Point charges q1=+2.00μc and q2=−2.00μc are placed at adjacent corners of a square for which the length of each side is 1.50 cm . point a is at the center of the square, and point b is at the empty corner closest to q2. take the electric potential to be zero at a distance far from both charges. part a what is the electric potential at point a due to q1 and q2? express your answer with the appropriate units.