In the single slit experiment, individual photons may be diffracted. The electric field of the em wave should accelerate charged particles in the screen (if it were a capacitor for example) but only one packet of energy exists. The photon may be absorbed after many wavelengths of light have passed over the charges particles. So, how can this work out? How do the charged particles know not to accelerate or do anything due to the wave until it is decided that a photon is to be absorbed?