Okay well I've started to study phys chem in school recently and I honestly can't get my head around what a real image actually is I've asked my teacher to demonstrated it but she's quite lazy, and then I got this sudden flashback of something I learned about renaissance art in primary school. Its the camera obscura painters used to use it or something I don't know but I just wondering does this explain what a real image is or am I just a idiot, here is a picture : http://upload.wikimedia.org/wikipedia/commons/a/a2/Camera_obscura_1.jpg
The best way to differentiate between a real and virtual image is, if you put a sensor (e.g., your eye) at the location of the image, does it actually see the image?
In the case of a virtual image, the place where it is when you draw the ray diagram, if you put your eye there, you won't see the image, because it's usually behind the mirror or whatever is creating it.
With a real image, if you put your eye where it is when you draw the ray diagram, you will definitely see something.
With a real image, there are continuous rays of "light" or other radiant energy propagating from the object point to the real image point.
With a virtual image, no rays go continuously from a point on the object to the corresponding image point on the virtual image.
Adding some of the side bar questions.
Yes you can photograph the virtual image with a camera pointed so as to see through the optical system producing the virtual image.
And as to why can't we see real images. Well in fact you do see a real image that is projected on your retina, and if you put a screen at the position of a real image, then you just look at the screen to see the real image.
With a simple magnifying gass type of lens, the real image will be inverted from the object, as if all the rays were passing through a pin hole. With the virtual image fromed by the same lens the image is not inverted. (the geometry of course is different from the real image case.)
Perhaps the easiest way to grasp the concept of a "virtual" image, is to simply look in an ordinary plane mirror. You are looking at a virtual image; possibly of yourself. Clearly you can point your camera at the mirror, and take a picture of yourself; and the camera too.
But you think there is another you, with another camera BEHIND the mirror. Clearly no light is coming from anything behind the mirror.