We have a certain system of lenses:

Lens 1: Focal length: 19.5 cm Lens 2: Focal length : 9.5 cm

I put an object 36 cm away from lens 1. Then, I put lens 2 between lens 1 and a screen. The separation from lens 1 and 2 is 10 cm.

I want to know where the image will be formed.

I get the right numerical answer. However, I cannot seem to understand why we get a "negative m" (ie. a virtual image).

When I draw the ray diagram by hand, I get that the image is real.

Can someone explain to me what is going wrong?

  • $\begingroup$ Oh boy! I see my mistake. Virtual images have a POSITIVE magnification. Someone please confirm that this is true for all virtual images! $\endgroup$
    – yolo123
    Commented Nov 23, 2014 at 22:53

1 Answer 1


Your m is negative because the image is inverted, not because it's virtual. Inverted images have a negative magnification. Virtual images are caused when the rays diverge from a point. A lens with a negative focal length typically causes them. A convex lens also forms a virtual image when the object distance is shorter than the focal length of the lens.


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