2 convex lens

First of all, please look at the following ray diagram first.

http://i718.photobucket.com/albums/ww188/physicsworld2009/physicsworld01Feb190958.jpg


Without the right lens, the image of the object at infinity will form on the focal plane of the left lens (with focal length = 3 cm), to form the image I'.

But now, since the right lens is added, I' acts as virtual image of this lens, for convenience sake, we call it lens R

So, the focal length of R, f' = 1 cm

Object distance from R, u = -(3 - 2) = -1 cm (since object is virtual)


Apply lens formula, 1/f' = 1/u + 1/v

1/1 = 1/(-1) + 1/v

Image distance, v = 0.5 cm

As the image distance is positive, it is on the right side of R.

Hence, the image position is 0.5 cm on the right of the right lens.