So in an experiment to find focal length of a concave mirror, first we had to estimate its rough focal length. I kept an object at point which was beyond the focus in front of the mirror and got its real image on the screen. But as soon as I removed the screen, I could even see an inverted image of the needle behind the mirror(i.e virtual image). How is this possible as the concave mirror always form a real image for an object kept at a distance greater than the focal length in front of it?
to see the real inverted image "behind" the mirror is kind of an optical illusion. The picture is still in front of the mirror, but your eyes can not find a frame of reference, so it "sees" the picture at the usual place you see it in plane mirror, a virtual picture would not be inverted. to help your ey to see the picture, at the place where it really is, place som kind of mauve frame instead of the screen, sometimes it even helps if you put just your finger beside the place the screen was before and you see the real picture in the air beside your finger.