 Now let's ask about the speed of this idealized wave and here we only consider waves of constant speed. How do we quantify the speed of the wave? The speed is the distance that a crest travels divided by the time that it takes to travel that distance. Now we've already have that from the graphs. Remember that the period is the time that it takes for the crest to travel its wavelength or the speed is the wavelength divided by the period. Basic characteristics of waves are demonstrated in a ripple tank. This is a shallow glass bottom tank of water. You generate ripples in the tank with a bar that is oscillated. This generates planar waves or wave crests that form a straight line across the tank that move the length of the tank. The ripple tank is illuminated from above. The light passes through the water onto a white sheet of paper placed directly below the tank. Now the light is bent as it passes through the water. A crest of water bends the light away from the crest while a trough focuses or concentrates the light. We'll discuss this later when we talk about light. But the end result is that on that paper the bright lines represent wave troughs and the dark stripes represent wave crests of the planar wave. Here is a ripple tank in action. Notice that they can still see images of the waves underneath the tank without the sheet of paper. Let's freeze the wave. The dark regions represent the crests, the white regions the troughs. You might recognize similar images on the bottom of a swimming pool on a sunny day say from dipping a finger into the surface and viewing the image created on the bottom of the pool. Those swimming pool waves aren't as regular as the waves generated in a ripple tank. Scientists study wave behavior in ripple tanks by observing how the dark and bright regions move. As the waves encounter an obstacle in its path, the waves' behavior can be observed by watching the movement of the dark and bright regions.