You are certainly familiar with the word optics. The optician in the mall is the man who sells eye glasses. And the glasses he sells are also optical instruments. The word originally came from ancient Greek and means "the science of sight and light". That's what makes optics so interesting: You can see it everywhere you look. That's right - without optics we would not be able to see! Without a lens in your eye you would only be able to recognize the difference between light and dark. But you would not be able to recognize what you were looking at and everything would be completely fuzzy. People have given a lot of thought to eyesight for a long time. A lot of attention has been given particularly to poor, or fuzzy eyesight. This is the reason they discovered how to use eye glasses and magnifiers nearly 700 years ago. These glass lenses make it possible to correct optical errors in the human eye. We will try this a little later. We cannot 'discover' the magnifier - it already exists. But we can copy it - and do a great deal more. We even show you how to build instruments for looking at the stars. The trick used here is called refraction. In addition to bending or refracting the light, it can also be refl ected. You may already know how a mirror works, but we are going to do something very special with it. We are going to 'bend' the light around a corner, we will even turn it around completely - 180°. You will be able to see people, who cannot see you. You can watch them without them even noticing. A famous philosopher once said "Where there is light, there is also shadow." He was right. When the moon shines in the sky, it is dark night for us here on earth - or what did he mean? And why can we sometimes see only half the moon, or only a sliver? If you have ever wondered about this, you will now have an opportunity to build your own fully functioning 3D planetary model. When you have tried out all the models, you will also be an expert in the field of optics. But regardless of how much you think you know - don't trust your eyes - at least not all the time. But we will go in to that at the very end.