> However, those intensity will be much smaller compare to the intensity of the outgoing > beam. That much is certain but why? In your illustration they just magically disappear : ) It's a great thing to teach but a shame to leave out something important. I understand if you cannot illustrate what actually happens but wouldn't you agree that it is important to mention that the model is far from complete? That reality is much more complicated. That more thought is required to understand it by the 'reader' than is immediately obvious. regards Dan