Voltage on a quadrant photodiode used with an AFM

[Phantoful]

Homework Statement

An AFM uses a quadrant photo-diode to measure the deflection of the laser beam. Sketch the voltage in quadrants A, B, C and D as the tip travels on top of a single carbon nano-tube resting on a flat surface when the AFM is operating on tapping mode.

Homework Equations

Not sure what to use, or if this is a conceptual problem.
λ= 1.2 / V^1/2
λ = h/p

The Attempt at a Solution

I know that the AFM (and the piezoelectric actuator) would have a changing voltage to account for different heights and such, but how does that relate to what voltage is on the photo-diode? How can there be a specific voltage for each quadrant, I'm not getting that.

 

[mfb]

Did you draw a sketch of the setup? Before finding relevant equations you'll have to understand what leads to a different current in the quadrants.