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I was reading geometric optics when I came across the concept of virtual objects. I found the concept counter-intuitive, as far as reflection goes. (I perfectly understand the formation of such an object when the light ray is refracted through a glass slab.)

Attaching the definition provided in my book:

Virtual Object defn

My confusion: A mirror forms an image when the rays originate from a certain object. So, how can an image of a virtual object form if the rays do not diverge from a particular point in the first place? What are some real-life examples of such a situation, as depicted by the figure below:

fig

I have checked answers to similar questions but my doubt is not cleared.

golden_hawk21
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2 Answers2

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So, how can an image of a virtual object form if the rays do not diverge from a particular point in the first place?

The image in your question shows a "virtual object". It does not show an image. You could create a real image from it, and in this case it would do exactly that in front of the mirror, if you finished drawing the rays.

This is just a definition, which is somewhat counterintuative, and it completes the set of real and virtual objects and images, the other three of which are somewhat more intuative.

  • Light diverges from a real object
  • Light appears to diverge from a virtual image (when an optic reflects or refracts it)
  • Light converges on a real image
  • Light appears to converge on a virtual object (except an optic gets on the way and refracts or reflects the light)

For real life examples its somewhat harder to come up with simple examples as a virtual object isnt often much use on its own. So it might be formed as an intermediary in a larger optical system. Or even in this example. If you have an imaging system with a fold mirror at the end, that mirror would create a virtual object, similar to your picture here.

Matt
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What "forms an image" (on your retina) is the lens in your eyes. Neither a real object nor a a flat mirror reflecting a real object can form an image. The virtual object behind the mirror is what we see because all the rays that are reflected by the mirror surface SEEM to be coming from that virtual object (that doesn't exist) if continue them behind the mirror surface.

I believe your diagram is wrong in one detail: it should show rays going in the other direction. This shows the actual optical path of a plane mirror surface: https://www.ck12.org/book/ck-12-physics-concepts-intermediate/r252/section/14.1/

Divergent rays from an object reach us and if we trace them back without the change in direction on the surface, then they seem to converge on the "virtual object" behind the mirror. Except that that's not what we call a virtual object. I defer to somebody else to explain why the convention was chosen that way. It's not obvious to me, either, at this point. I am sure there is a good reason for it.

FlatterMann
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