What is an optical isomer?

An optical isomer is when a molecule has a 'non-superimposable mirror image'. This means that if you draw out its reflection, it is not the same as the original. A good example in daily life would be feet - they are mirror images of eachother but your left foot is clearly different from your right foot - so you can't put your right foot in your left shoe. The same happens with molecules.A standard example a A Level is that any carbon with 4 different groups attached is an optical isomer. If you draw out it's mirror image, you can see that the two are actually different. (Here I would get them to draw it out and confirm it for themselves - resorting to building 3D molecules if necessary).