Waves: How we know what's inside the Earth

Homework tonight:  Read p. 354-361; p. 361#1-9.
Also, due Thursday:  Read p. 362-369; p. 369#1-7.
On the p. 369 assignment, change two values in problems #6 and 7, and make them IN PEN in your textbook please:
#6 make the p - wave speed 8 km/s
#7 make the s - wave speed 4 km/s

Today in class:  sharing of ways to show that the Earth is round.  Very creative solutions were shared by many, bringing up lots of engaging discussion.
After the "Round vs. Flat Earth" evidence, we:
--> took notes on waves:  Speed, Refraction (bending), and types, as well as the implications of Earthquake waves in giving us evidence of the Earth's interior composition and nature.
A basic outline of the notes follows:
Wave Speed:  Waves move at different speeds based on the medium (material the wave is moving through).
If a wave encounters a boundary between one medium and another, it may change speed (+ or -).
If this boundary is encountered at a right angle, the wave is largely unaffected in direction.
If the boundary is encountered at an angle other than 90 degrees, the wave will be bent, or refracted:
Good diagrams here:  http://www.ux1.eiu.edu/~cfadd/3050/Ch16R&R/RefR.html

This occurs with all waves: light, radio, sound, seismic, water, etc.  How is this at all connected to Earth Science?  The image below illustrates seismic body waves moving through the Earth away from an earthquake epicenter.  If the Earth's interior was uniform, waves would travel in straight lines.  They don't; they curve back toward the surface, and in some places curve multiple times.  This tells us that the Earth's interior has different densities and temperatures...  As for the Shadow Zones at the bottom, read on.

There are two main types of waves; longitudinal (forward and back) and transverse (side to side):

In a longitudinal wave, the medium vibrates in the same direction as the wave moves (parallel).  In a transverse wave, the medium vibrates perpendicular to the wave's direction of motion.
In Seismic waves, these waves both exist.  Longitudinal waves move fastest through the Earth and are called Primary waves, or P waves, because they arrive at distant locations first.  Transverse waves move a little slower and are called Secondary waves (S waves).  Here's the really weird thing:  S waves don't travel through the center part of the Earth at all; they are absorbed.  The best explanation for this to date is that S waves don't travel through liquids, and this region, now dubbed the "outer core" is probably liquid.