I quote from my textbook, " Consider two vertical slits S1 and S2 placed parallel to each other, and a string is passed through them. The end B is fixed and A is given jerks perpendicular to its length. If on rotating slit S2 till it is perpendicular to S1 the amplitude of vibrations becomes zero then the wave is a transverse wave, otherwise it is a longitudinal wave."

I understand how this works for mechanical transverse waves, but not for light. The electric and magnetic vectors do not have spatial extension, though it is shown that way in animations. The oscillating vectors just show the varying magnitude of the fields at points on the ray of light. Otherwise its like saying a physical arrow(velocity vector) extends in front of a moving body whose size depends on the body"s velocity.

So my question is why should the amplitude of light be affected when the oscillations have no spatial extension(considering just one ray of light passing through the slits)?