Preparing for Lower Sixth Physics #1: Progressive waves

This is the first in a series of posts about topics for the Lower Sixth Physics course next year. Links to other resources were posted on Tuesday 24 June. Look them up!

Look at the 3 photographs above. They show the same wave as it breaks on Silloth beach. You can see that the peak of the wave moves from left to right. When that happens, we call it a PROGRESSIVE WAVE. It carries energy from one place to another.
There was a dispute about how to draw this on diagrams. The Dutch physicist Chritiaan Huygens drew lines along the peaks of the waves. These are called WAVEFRONTS. Isaac Newton wanted to show the forward motion of the waves, which is impossible to see in a still photograph, so he drew arrows to show the direction of motion. These are called RAYS. In the photograph below, the dotted red line shows the wavefront. It is what you normally think of and see. The yellow arrow shows the ray.