Describing Diffraction

We're half way through our adventure, Young Physicist. You've come a long way so far! Let's keep going.

I can see you getting ahead of yourself. Diffraction, unfortunately, is not a lot like reffraction. In fact, it's not like it at all! Sorry. Oh, no. Don't get upset. Diffraction is easy, I promise!
What is diffraction? Oh, well, diffraction is the spreading of waves into a region behind a barrier. Also very important to diffraction is Huygens' Principle which states that any point on a wave front can be treated as a point source of waves. You see, a wavefront can be broken up into little "wavelets" which form a new wavefront. This is a lot easier to understand through a diagram.

Because wavefronts are made up of even smaller wavelets, when a wave passes through a small opening, it can change its shape and bend. Notice that in the diagram, the one on the left has a big opening between the barriers. The waves on the other side of the barrier bend only a little bit around the edges. The diagram on the right shows a smaller opening between the two barriers. The waves on the other side of this one are very circular.

http://upload.wikimedia.org/wikipedia/commons/b/bb/Water_ripples_Diffraction.png

Why? Good question! If the opening is very, very small, only one wavelet can squeeze through, right? Right! Therefore, when the wavelet breaks off from the rest of the wavefront, it forms its own wavefront. This wavefront is now circluar and made up of even more wavelets that spread out in a circular fashion. If the opening is large, nine or ten wavelets can fit through. When the wavelets are together, they keep the wavefront straight. The ends of the new wavefront curve slightly because they are the ends of the wavelets.

Although you may not realize it right away, this actually does happen in real life! Who would have thought? Perhaps one summer evening you were outside playing catch with your beloved neighbors. The windows of your house were open to let in the cool, most needed breeze. All of a sudden, you hear your mom talking! It sounds as if she's at the window speaking directly to you! How could this be if she's not actually right there? Diffraction, young one, diffraction. Her voice travels through the kichen and squeezes itself through the open window, only to make it into your ears.

Interesting, huh?