Module 8—Mechanical Waves
Lesson Summary
As you worked through this lesson, you should have developed partial answers to these questions:
- What is the difference between a wave and a ray?
- What happens when a wave encounters a boundary?
- What is Huygens’ Principle? How can it help to understand wave reflection?
A ray is a line that depicts the direction that a wave travels. It is always drawn perpendicular to the wave, showing the wave’s direction of travel not the wave itself. Ray diagrams are a convenient tool to use when studying waves because they represent the direction of motion and can be used to predict the precise angle of reflection from a smooth surface.
When a wave encounters a boundary, the law of reflection is applied such that the angle at which a wave approaches a barrier is equal to the angle at which it is reflected. Expressed as an equation, it is 
Any wave front can be thought of as a series of points. Each point acts as a source of tiny secondary waves, called wavelets, which propagate outward in concentric circles at the same speed as the wave itself. The line tangent to the wavelets represents the wave front. This idea, called Huygens' Principle, can be used to explain reflection.
Lesson Glossary
angle of incidence (θi): the angle that an incident ray makes with the normal line
angle of reflection (θr): the angle that a reflected ray makes with the normal line
incident ray: the ray that depicts the direction of the wave front that is moving from the point of origin toward the barrier
normal line: an imaginary line that is perpendicular to the boundary
ray: a line perpendicular to the wave front depicting the direction the wave is moving
reflected ray: a ray that depicts the direction of the wave front moving away from the barrier
reflection: a change in direction when a wave strikes and bounces from a surface
wavelet: a secondary wave