Is light deflected by external electric and magnetic field?
Looking at the classical electromagnetic wave with E and B fields propagating perpendicularly to each other and the direction of motion. These fields do not carry charge, and it is only charge that is deflected/senses electric and magnetic fields. Classically, there can be superposition of two waves which show interference patterns, but superposition is not interaction. The classical electromagnetic wave interacts only when it sees charges (and,possibly, magnetic dipoles). Another way of looking at it is that the dielectric constant of the space between the capacitor plates does not change whether the capacitor is charged or not.
If one goes to the photon level, firing individual photons through the opening of the capacitor plates: photons as neutral will change direction only if they interact. There exists a small probability that a photon meeting an electric field may interact with the electromagnetic interaction, but the probability is tiny as the relevant Feynman diagram has at least five coupling constants. The four of this diagram + one extra because the interaction with the field is with a virtual photon which means an extra vertex.
Light is not deflected when it passes through a capacitor.
Light is classicly an electromagnetic wave, and visualizing it as the water waves in a pond might help you solve this cognitive dissonance.
In the finite region in which there is a electric field generated by the capacitor, the electric field of the wave and the previous one interfere through superposition (because, as you might have studied, Maxwell equations are linear), but after that, since the wave hasn't got charge anywhere in it, there is nothing the Lorentz force can interact with, and thus nothing is accelerated, nothing is deflected, and the wave exists the capacitor as it entered it.
This is of course consisten with the quantum mechanical notion that light is made of photons which have no charge, and therefore cant be deflected by electric fields.