(2) narrowing; (object distance > 2f, f < phase distance < 2f)
(3) 30
∵ parallel light source
∴ light rays pass through the focal point
and ∵ spot size is equal
∴ the focal point to the two light screens at the same distance
. That is, at 80cm
That is, the focal length is 80-50=30cm I hope to accept O(∩_∩)O Thank you