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## Homework Statement

Electrons which have been accelerated from rest through a potential of 500 V pass between two parallel plates that are 6.0 cm long and 2.0 cm apart, with a potential of 150 V between them. The screen is 20 cm beyond the end of the charged plates. What is the size of the deflection as seen on the screen?

## Homework Equations

## The Attempt at a Solution

I have calculated that the deflection of the electrons while passing through the region between the plates is 0.014 m. This is correct, same answer as the answer key.

To calculate the size of the deflection as seen on the screen, I would say that

D/0.014 = (20+6) / 6

as there are similar triangles

But instead the answer key says it is

D/0.014 = (20+3) / 3

Why does it only take half of the length of the charged plate to do this proportionality between similar triangles and not the whole side?

I know that the deflection is not significant until it reaches maybe half the plate. Is this the reason why it only takes half?

Any help really, really, greatly appreciated.