Now, we want to start talking about images produced by different types of lenses and mirrors: convex and concave lenses – and a bit later, by concave/convex mirrors. Optical images are of two types, called real and virtual. Real images are those one can see using a screen (example? What you see now on this screen, is a real Image). Virtual images produced by optical devices cannot be projected on screens – we can observe them only by eyes (actually, photographic and video cameras can also “see” them – but such devices are, in fact, “artifi- cial eyes”, they work very much like human eyes).

For better understanding of what a virtual image is, let’s talk for a moment of how our visual perception works. Let’s first think of a point source of light. Real point sources don’t exist, but a tiny source, e.g., a miniature LED, or a single pixel on a computer or TV screen. A point source delivers a beam of diverging light rays.

Now comes an observer – she looks at the point. Diverging rays from the source enter her eye. Signals are sent from the eye to brain – we don’t want to discuss now the details of this process – but the result is that in the brain registers an “image” of the point. What the brain registers This is still visual perception of a real object, not observing a virtual image!

Add another “point source” – now the brain registers two tiny bright dots

Actually, such a “point source” does not need necessarily to emit “its own” light. One of those tiny pins with color heads may also be a good model of a point source. The light that comes from it is scattered light from another source. But it’s also a beam of diverging rays, and this is all that matters. In fact, most objects we see in our everyday lives are not light emitters, but “scatterers” of light they are illuminated with. Examples of exceptions from that rule are stars, all kinds of signal lights, or computer and TV screens.

But most objects we look at are not points, but rather sizable objects! So how does our visual perception work with such things? Let’s look here, using a microscope

This is perhaps a slightly “oversimplified” picture – however, it is certainly correct to think of any sizable object as of an “array” of miniature “light scatterers”, or “point light emitters”.

Let’s return to the initial picture: there is a “point source”, and a beam of divergent rays emerging from it. But now, imagine that a witch arrives! She rem- oves the light source. She replaces it with a “magic black plate”. It emits light from its sur- face – in such a way that the rays form a diverging beam.

We want to examine the geometry of the rays, so we plot their extensions on the other side with dashed lines. And it turns out that they all converge at one point! So, the geometry of the rays on the right side of the plate is the same as in the original beam produced by the point source! Let’s then ask a question: what would the observer see, looking at the plate from the right?

This is a picture shown at the beginning – Here the obser- ver sees a real object. And here there is no point at all, but the geome- try of the rays entering the ob- servers eye is identical, so her visual impression is exactly the same. But what she sees now is a virtual image.

OK, but magic was involved in that story. But what about our real word? Do such “magic plates” really exist? The answer is YES! Not exactly like the plate we talked about, but plates producing a very similar effect. And you know them well, because what I’m talking about is our good friend, the FLAT MIRROR! A good Web document about mirrors