Answer:
E = 8.5 * 10^6 V/m
Explanation:
In general we have the following relation between the Electric Field and the Elecric Potential:

Due to the vector nature of the electric filed, we can only know the mean Electric field E across the membrane, and take it out from the integral, that is:
E = (ΔV)/L
Where L is the thickness of the membrane and ΔV is the potential difference.
Therefore:
E = 8.53933*10^6 V/m
rounding to the first tenth:
E = 8.5 * 10^6 V/m
The 'strength' of the electric field is the force on 1C of charge at that point.
At this 'certain location', the field is 40/5 = 8 newtons per coulomb = <u>8 volts</u>
It's true answer that interference is the evidence of the particle nature of light..