Real and Virtual Images

Every image is just a record of where light rays end up after bouncing off a mirror or bending through a lens. If you trace the actual paths of rays leaving one point on an object, sometimes those rays genuinely cross at another point in space. Put a screen there and you catch a sharp picture: that's a real image. Other times the rays never cross at all. They leave the mirror or lens spreading apart, as if they came from some point behind the mirror or behind the lens, but nothing physical is happening there. Your eye still catches those spreading rays and your brain, which always assumes light travels in straight lines, back-projects them to a point of apparent origin. That's a virtual image.

Ray diagrams make this concrete. A converging lens making a real image: rays leaving an object point, refracted by the lens, physically cross on the far side, and you can draw solid lines meeting at a point. A virtual image from the same lens, with the object close in, has rays leaving the lens still diverging; you draw dashed lines backward through the lens to show where they seem to meet. Solid lines that cross mean real; dashed lines that only seem to cross mean virtual.

This distinction carries forward into every calculation with mirrors and lenses. The sign conventions for image distance exist entirely to track real versus virtual: a positive image distance means a real image on the far side, a negative one means virtual, on the same side as the object.