Question

A 2.0 x 2.0 -inch slide is placed before a converging lens 21 inch in front of the focal point. If the focal length is 20. inches, how large is the image that is projected on the screen?

______ inches

Answer 1

These are the choices

42 x 10.
40. x 40.
420 x 420

Answer 2

First choice is logically wrong.
from the equation Df focal length, Di + image length and Do slit length from origin point
\[1/Df = 1/ Di+ 1/Do \]
Thus Di = 420 inch from origin point.
Magnification formula(you can prove it from similarity of triangles) is
\[m = h'/h = Di/Do\]
,so \[h' = Di * h / Do = 420 *2/21 = 40 inch.\].
The answer is the last one.
you can check it with this flash.
http://phet.colorado.edu/sims/geometric-optics/geometric-optics_en.html
Have fun :D