Thank you for your site I can understand the ray diagram and why the image can be 3D but I don't understand why the image can be observed without screen. When we form a real image just by convex lens or concave mirror, the image can't be observed, unless we sit on the image point. At least, we won't see something aerial like this toy. For the java, I have some confusions, when the distance between mirror and the object is 0, the image should be formed on the focus. For the red line, do the different ray paths exsist? If the ray paths like the java showing, when there is not the upper mirror, the rays go away, so how can the image be formed at focus?? [img:fe5c1b8171][/img:fe5c1b8171] There is on thing I can sure, the total image must be formed by lot of views of images. thus it is a combination of mutliimage. From this, I made an assumption that when mor than 1 image are overlapping, we may observe it without screen. Is that possible?? I have made an assumption that is because of interference of muti-foucsing point but my lecturer said that is not true, I have no idea, no direction to analysis that now............. thanks again for your answering~~