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My book answer is: There is only one convex lens in simple microscope which produces virtual,erect and magnified image when object distance is less than focal length.IN this case,real rays are diverged more.Thus, eye is to be placed very close to lens so all rays diverged by lens enter eyes and hence image will be brighter.

How can we get diverging rays from convex lens??

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enter image description here

When used as a magnifying glass a convex lens bends a ray towards the principal axis.
Ray $AP$ is initially parallel to the principal axis but changes direction and crosses the principal axis at the focal point, $F$.
The angle between rays $AP$ and $AO$ which is $PAO$, is larger than the angle between rays $PF$ and $AO$ which is $FA'O$, so the angle between the rays has been reduced (ie made to converge), but they are still diverging.

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  • $\begingroup$ This picture is a bit confusing, because the real image is inverted, but the viewer (at the close-in position, sees a virtual image which is not inverted. Perhaps you could elaborate on that point? $\endgroup$ Dec 2, 2020 at 14:50
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    $\begingroup$ The image in the picture is virtual with the real rays moving to the left appearing to come from the virtual image which is upright and magnified. $\endgroup$
    – Farcher
    Dec 2, 2020 at 15:49
  • $\begingroup$ Ahh -- ok, you drew the whole picture "backwards" -- the standard is to put the physical object on the left and the viewer (and/or image plane) on the right, so that all optical rays are moving from left to right. That's what threw me off. $\endgroup$ Dec 2, 2020 at 19:30
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A convex lens used as a simple magnifier has the object brought close to the lens (within the focal point) where it subtends a large angle. The virtual image subtends the same angle and may occur at a large distance where the eye can easily focus. This focus can be maintained with the eye close to, or not so close to the lens. To achieve this increased angular size, without the lens, the object would often have to be brought within the near point of the eye.

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