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Reflection occurs when a wave hits a boundary of some sort and bounces off of it, staying in the same medium. This is different from refraction in that the wave does not cross the boundary into a second medium.

Ray_Diagram_2.PNG

When looking at a situation of reflection, a normal line is often constructed perpendicular to the boundary to analyze angles. (See image above.) The angle between the incident ray (the wave that first hit the boundary) and the normal line is the angle of incidence. The angle between the reflected ray (the wave that has just been reflected off the boundary) and the normal line is the angle of reflection. These two angles are always equal. (This is referred to as the Law of Reflection.) You can easily verify this by looking at the real world: What happens when you shoot a ball in a game of pool and it bounces off the wall? What happens when you look at a mirror from an angle that isn't 90 degrees- what do you see? The way reflection works for objects and light waves, it does for sound waves as well.

That's all there is to reflection for sound waves, which are longitudinal. But just for the sake of accumulating knowledge, we'll tell you about the more complicated concepts that have to do with transverse waves: There are different ways for a wave to reflect, depending on the boundary. When a wave hits a fixed boundary, one that does not move or allow the end of the rope/spring to move, the reflected wave is "inverted." This means that every crest that originally hit the boundary is now coming back as a trough, and vice versa. (Visual below.)

Inverted_Reflection.PNG

When a boundary is free (such as when you create a wave in a rope laying on the ground, free to move at the end you are not holding), the opposite is true. Crests come back as crests, troughs as troughs. This type of reflection is called "erect." (Visual below.)

Erect_Reflection.PNG

The reason why sound waves cannot do this is because they cause the medium to vibrate in a different plane. A longitudinal wave in a spring, for example, looks like a bunch that travels along the spring. This is also called a "compression wave" and when one bounces off a boundary, fixed or free, it reflects as a compression wave with no difference. Thus, inversion is not applicable in the case of sound.

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