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?

Question

anonymous · Answer

Frist you need to determine where the image is formed.  To do that, use the thin lens formula:$$\frac{ 1 }{ s _{i} }=\frac{ 1 }{ s _{o} }+\frac{ 1 }{ f }$$where si is the distance of the image from the lens; so is the distance of the object from the lens; and f is the focal length  of the lens.  Note that distances to the left of the lens are negative.

Then you can find the size of the image using the formula for magnification:$$m=\frac{ s _{i} }{s _{o} }=\frac{ h' }{ h }$$where m is magnification; si and so are as given earlier; h' is the image height; and h is the object height.

anonymous · Answer

So I know f, but do I have si or so?