Parallel rays entering from the left in the photograph at the left above are focussed to a point by a plexiglass double convex lens. At the right above the same rays are shown displaced upward by rotation of a rectangular plexiglass block, but without the focusing lens.
Q: What will happen to the transverse (vertical) position of the focal point when the rectangular block is rotated while the lens remains at the same point? Will the focal point be displaced upward, be displaced downward, or remain at the same lateral position? A: The focus will remain at the same point, as seen in the photographs below. In fact, all rays parallel to those shown in the photograph will focus at the same point, in the absence of aberrations. This is a fundamental property of how lenses act to focus light.
The parallel rays are formed by a second double convex lens out of the photograph to the left.