The figure below shows three parallel horizontal light rays moving from left to right, passing through a rectangular block of plexiglass and focused to a point by a plexiglass lens. The focal point is marked by a large dark dot.

If the rectangular block is rotated counterclockwise, the rays will be displaced upward, but continue on parallel to each other and horizontal, as seen in the figure below with the lens removed. Note that the vertical position of the focus (the dot) is along the lower of the three rays.

Now suppose that the lens is left in place when the rectangular block is rotated. What will happen to the position of the focus?

 

• (a) The focus will move UPWARD.
• (b) The focus will move DOWNWARD.
• (c) The focus will remain in the same position.

Click Read More after January 31st, 2014 for the answer

 The answer is (c); the focus will remain at the same point. This is shown in the pictures below, comparing the case with dislacement of the rays and the original focus where the block is straight.

 

In fact, the focal point for all parallel rays entering the lens is the same - the definition of a focal point.

One thing that makes this problem complicated is that if the vertical position of an object is changed the vertical position of the image IS changed. Although it appears that this is the case, moving the parallel rays upward is not equivalent to changing the vertical position of the object.