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My Physics teacher made the statement in a recent class.

"A concave mirror always forms a real image of a virtual object"

But, what did he mean by a virtual object? What does this statement exactly mean?

PS : I have just started learning geometric optics.

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A real image is formed when the rays converge in one point. That means, if an object emits light, and there is an optical system that makes those rays converge, then there is a real image where those rays converge.

On the other hand, a virtual image is formed if the rays do not converge, but their prolongations do. This usually happens if the rays diverge, but if you extend them backwards, their extensions converge.

Real images can be projected on a screen, but they cannot be seen with the eye (you can see them on screens, not looking directly to them), think of a projector. On the contrary, virtual images are not projected, but they can be seen with the eye (think of a magnifying glass)

These are important basic concepts. Once you understand them well, it follows that, if you have a system made of several instruments, the image created by the first one acts as the object of the second one.

So if the instrument 1 makes a virtual image, that virtual image is the object for the second object.

So you can have a virtual object for a mirror, if something's creating a virtual image before the mirror.

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  • $\begingroup$ One error: virtual objects can most certainly be behind mirrors - just continue the raylines as though the mirror were a lens. Also consider convex mirrors, e.g. physicsclassroom.com/class/refln/Lesson-4/… $\endgroup$
    – Carl Witthoft
    Commented Aug 5, 2020 at 14:01
  • $\begingroup$ You're right, I mixed it with real objects. Edited. $\endgroup$
    – FGSUZ
    Commented Aug 5, 2020 at 14:49
  • $\begingroup$ "if you have a system made of several instruments, the image created by the first one acts as the object of the second one." Did you mean something like 2 plan mirrors parallel to each other and facing each other ? So if there is an object in between its image by the first mirror acts like an object for the 2nd one ? $\endgroup$
    – Koustubh Jain
    Commented Aug 5, 2020 at 17:13
  • $\begingroup$ Well, yes, that's a particular case, true. However, it's not that common. I was thinking about two lenses (one after another), or a lens before a mirror, but yes, any combination is valid $\endgroup$
    – FGSUZ
    Commented Aug 5, 2020 at 23:24
  • $\begingroup$ This is a picky point, but words like "always" are a warning sign. A real image is not always formed. For example, if the radius of the concave mirror is very long (nearly flat), then the mirror does not cause the reflected rays to converge enough to form a real image. It might only change the divergence of the rays slightly, which would cause the location of the virtual image to shift. (But it still leaves a virtual image). $\endgroup$
    – JB2
    Commented Aug 7, 2020 at 22:12

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