Physics/ lens question

Question

An object is placed upright on the axis of a thin convex lens at a distance of four focal lengths (4 f) from the center of the lens. An inverted image appears at a distance of 4/3 f on the other side of the lens. What is the ratio of the height of the image to the height of the object?

Answer 1

1/f = 1/do + 1/di where do is the object distance and di is the image distance

Magnification is -di/do so you have -(4/3f/4f) =  - 1/3  

So the ratio of the image height to the object height is 1/3 and the image should be inverted.  You can prove this easy.  Take a magnifying lense outside with a piece of paper on a sunny day.  Hold the piece of paper up in front of the lense like a screen infront of a projector.  Look for some noticable object in the distance behind the lense like a tree or a house.  When the paper gets to the focal length of the lense the image of the tree will appear on the paper, inverted (upside down) and reduced in size.