Refraction and the Shadow Zone

Definition of Refraction The change in direction of a wave due to changes in its velocity as it passes from one medium to another

Questions What is a medium? What is a boundary line? What happened to the angle when the wave crossed over the boundary line. Why did the wave travel at different speeds?

Part C: Pencil Refraction

Activity Obtain a pencil, glass beaker, and fill it with water. Place the pencil in the water. Observe where the pencil and water meet. This needs to be done at eye level. Observe the pencil in the oil and water beaker at the Lab station. Observation Record your observation in your notebook.

Questions 1.Draw a picture of what you observed. 2.Do you think that light waves travel through the water as they do through the air, or at different speeds? Explain. 3.Relate the pencil in the water experiment with the bending of waves through the boundary line. 4.Imagine a liquid in which light waves travel twice as fast as they do through water. Draw a picture of what the pencil would look like if it were put into this liquid. Explain your drawing.

Example

Why the pencil looks bent The pencil is not bent, yet appears bent Light travels at a constant speed Light travels at a constant speed through air When the light ray or wave enters water it slows down (water is denser than air)

Why objects in water may be at a different location than what you see…

Our eyes…

A prism…

Part D Refraction of Earthquake Waves on the Earth

Goal Using the diagram of the Earth you will show how direction changes when waves go through different layers of the Earth.

Diagram of the Earth Large circle represents the earth. Small circle represents the edge of an inner part of the Earth where earthquake waves move faster than the outer part. Black dot near the right hand edge of the large circle represents a place in the earth where earthquakes happen (focus).

Background Earthquakes sends seismic waves in all directions. Lines extending from the black dot show the start of earthquake waves. Your challenge: show how these waves change direction as they go through different layers (medium) of the Earth.

Directions Using a straight edge and a pencil extend the lines through the Earth to the other side….but remember – Some of the lines will go through the Earth without hitting the inner circle. – Some of the lines will hit the inner circle (boundary line). – The lines that go into the inner circle also come out of the inner circle. The wave will hit a boundary again. – Remember part C. “The refraction of waves” when you are doing this activity.

Part D Question 1.Describe the pattern of waves when they go into the inner circle. 2.What happens when the waves leave the inner circle. 3.If an earthquake occurs in California, will they feel it on the other side of the planet? Explain. 4.Scientist use this pattern to argue that the Earth has a core. They claim that it showed that seismic waves pass through a zone with different wave speeds. Does this make sense? Why or why not?

Seismic Waves Copy in Your Notebook Seismic waves change speed as they travel from one layer to another In other words, they bend or “refract”

When an earthquake occurs, the seismic waves can be felt at different parts of the Earth However, there is one area of the Earth where the earthquake is not felt. We call this the “shadow zone”

The existence of the shadow zone has led to the discovery of the core The only way a shadow zone can be present is if the seismic waves refract Seismic waves will refract when they enter a new material or layer and change speed