中三物理:點解凹鏡只會成虛像?

為何凹鏡只會成虛像, 而且虛像永遠都會比實物小???
I presume that you refer to convex lens (凸透鏡) and concave lens(凹透鏡), but NOT convex mirror (凸鏡) and concave mirror (凹鏡).
Without using detailed mathematical analysis with the thin lens equation, one would expected that for a concave lens, rays are diverged after passing through it. Since a real image is formed by the actual intersection of light rays, rays diverged out by the lens will not be intersected on the other side of the lens. Hence no real image is formed.
凸鏡成的實像點解一定系係鏡的另一面??
Real images are formed by actual intersection of light rays. The rays are only brought to intersect because of the convergence by the lens. It is thus apparent that any real image must be formed on the other side of the lens.
In the actual fact, it can be regarded as a general principle for lenses that any real image must be (i) formed on the opposite side of the lens, and (ii) inverted.
Likewise, a virtual image must be (i) formed on the same side of the lens as the object, and (ii) erect (i.e. not inverted).

The (virtual) image becomes smaller than the object just follow from geometry that any divergent ray, after extended backward, must meet with other rays at a distance between the object and the lens.