Suppose you have extremely sensitive photographic film and you can expose it in an optical system for a very long time. Will you be able to record a virtual image on film?
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No. Remember that images look like objects because light comes from images in the same way that it comes from objects.
Consider a point source of light. (More complicated objects can be treated a set of point sources.) If a lens, mirror etc. produces a situation where rays of light pass through a point and then diverge in the same way that they diverge from the source then we say a real image is formed at that point. You can take some photographic film, put it there and one point on the film will be exposed - you have photographed the image.
A simple scenario that produces a virtual image is plane mirror. Rays of light diverge from the source and reflect off of the plane mirror is such a way that they diverge as if the were all coming from a point behind the mirror. We say this is where the virtual image is located. The conventional way to show this on a ray diagram is to extend the reflected rays behind with mirror as shown with a dotted line.
Unlike with a real image, if you were to take some photographic film and put it behind the mirror at the point where the rays of light seem to diverge from the film would not be exposed no matter how long you wait because there is actually no light there. It's probably in the middle of a brick wall or a dark medicine cabinet.
You can photograph a virtual image just by pointing your camera at the mirror and focussing at the right distance behind the mirror. Now, when you photograph a virtual image you are actually forming a real image on the film. The same could be said if you simply see a virtual image - your eye produces a real image of the virtual image on your retina.
