I do know the difference between real and virtual images, cf. e.g. this Phys.SE post. I would like to know the difference between the real and virtual objects. I need a real life example.
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Your diagrams say it all. Real objects are points from which light diverges. A normal eye can take these divergent rays and converge them to points on its retina.
Virtual objects are points towards which light converges. If there were no eye or optical instrument in the way, there would be real images at these points. But suppose you place the pupil of your eye at S2 (where that blue thing is in the top right hand diagram). A normal eye wouldn't be able to accommodate (focus) these converging rays, because in everyday life you simply don't have rays converging to a point. However, the action of certain optical instruments can sometimes be analysed using the notion of a virtual object.
The concept of a virtual object is quite a sophisticated idea and I would expect students to have met the thin lens equation, $\frac{1}{u}+\frac{1}{v}=\frac{1}{f} $, before meeting virtual objects. So here's a simple exercise that should help with the idea…
An illuminated object is placed at the 0.0 cm mark on a metre rule, a converging lens of focal length 10.0 cm at the 15.0 cm mark, and another such lens at the 25.0 cm mark. All three are co-axial.
(a) Show that the first lens produces a virtual object for the second lens. (b) Determine the position along the ruler of the real image produced by the second lens.
Philip Wood
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