13. Collisions with boundaries: Reflection.

Waves reflect from a boundary in two basic ways depending on whether the boundary is "hard" or "soft". In the case of waves on a string a "hard" boundary is where the string is firmly attached and a "soft" boundary is when the end of the string can slide up and down. The string in this animation is simulated as a row of individual masses connected by invisible springs.
Note: Animations may take a few seconds to load.

            

Questions:

13.1. First look at how a Gaussian pulse reflects off the two different boundaries. Experiment with the two cases using the buttons above. How is a pulse reflected from a fixed boundary different from one reflected from a free boundary?

13.2. Now use to two buttons below to see what happens when a sine wave hits the two types of boundaries. What is the end result in these cases?
13.3. Although the reflecting sine waves in both cases interacts with the incoming wave to form a standing waves there is a slight difference between the two. Which case has a node at the boundary and which has an anti-node at the boundary?

A wave reflected from a stiff or fixed boundary is said to have a phase shift of 180o (or p radians). This means a pulse will invert itself on reflection and the first anti-node of a standing wave will occur 180o from the boundary. If the boundary is soft the first anti-node occurs at the boundary. As we will see shortly, other kinds of waves also experience a phase shift on reflection from some kinds of boundaries. For example when light in air reflects from a material that is more optically dense (such as a glass) there is a phase change of 180o but when light in glass reflects from a glass/air boundary there is no phase change since the light is going from a more optically dense material (glass) to a less optically dense material (air).

Credits.
Go To: IUS Physics Top Page.
Contact Dr. K. Forinash, for comments/suggestions/corrections.