# E field of parallel plates with metal inside

I was asked to determine the electric field in the spaces between the copper bar and the plates. The voltage across the plates was 30 V, so naturally I’d use $E=\Delta V/d$ to determine the field. However, the answer used 0.50 mm as $d$, rather than 1.00 mm. I do not understand why that is the case. I interpret 0.50 mm as being the $d$ corresponding to the total sum of empty space, but when you place conductors in electric fields, don’t they have no effect?

I learnt from my books that if you place a hollow metal container in a uniform electric field, it will seem as if a chunk of field had been removed at the container’s interior, but otherwise everywhere else the field remains the same—the field polarizes the walls of the container and causes the field to continue on the other side. See the diagram below.

So I thought I would put 1.00 mm as $d$ as if the metal were of no consequence, but it was wrong. Why?

(Question from here, 3c(iv).)