Measuring image distance of a virtual image How to a measure image distance of a virtual image without calculation?
It is impossible?
To measure it, we must have a real image to measure the image distance with a screen and a ruler.
I saw it somewhere that they added some other optical parts and turned a virtual image into a real image ( actually a real copy of that virtual image ).
How is it possible?
 A: Correct. The only way to experimentally determine the position of a virtual image is to convert it to a real image and calculate where the virtual image was.
This is simpler than you probably think. For example if you can see the virtual image that means the lens in your eye is focussing the image onto your retina. Replace your eye by a glass lens and a paper screen and the real image should be focussed onto the paper screen just as it was on your retina. Then just use the lens equation to work out the distance of the virtual image from the lens.
A: There are some special circumstances where the position of a virtual image can be measured.
Assume you have a real object located, say, 2 meters in front of a plane mirror.  The virtual image is located 2 meters on the other side of the mirror.  Of course, no light from the object actually goes there.
However, let's further assume that the mirror is a partially reflecting one;  a typical interrogation room with observation room setup.  The object and we, are in the observation room, and the virtual image, is in the interrogation room, even though the rays of light responsible for the virtual image never enter the interrogation room.  Now, some of the light from the object does penetrate the mirror, but we need to ignore it;  it never comes back, nor does it forms any type of image for us. (But note the use below!).
But we, in the observation room, will simultaneously see the real objects in the interrogation room, and the virtual image in the same room.  The two will be superimposed in three dimensional space.
A friend in the interrogation room can help us.  He could hold up an extended tape measure and we could use parallax  (moving our head back and forth)  to determine what point on the real tape measure coincides with the virtual image.
Our friend could help us in another way.  With the right lighting, he will be able to see the real object in the observation room (Remember those rays from above?).  He will also see a virtual image of himself, and of his tape measure, in the observation room, courtesy of his side of the partially reflecting surface.  He will be able to do the same parallax tests and confirm our determination of where on the virtual tape measure image the real object is located...
