12.5. Refraction of Mechanical Waves http://www.ck12.org
Consider the image sketched above. Suppose that the waves represented here are water waves. The wave crests
are represented by the black lines in the image. As such, the distance between two consecutive black lines is the
wavelength. Let the red line represent a transition from deep to shallow water. This transition is called themedia
interface. As the waves hit the boundary, the waves slow down. The right side of the wave reaches the boundary
before the left side of the wave, causing the left side to catch up and the angle of propagation to change slightly. This
change in direction can be seen in the yellow line, which is slightly angled at the boundary.
The refraction of waves across boundaries operates similarly to the method by which tanks are steered. Tanks do
not have a steering wheel. Instead, they have an accelerator to produce forward motion and separate brakes on each
tread. The operator uses brakes on both treads at the same time in order to stop, but brakes on only one tread to turn
the tank. By braking one side, the operator causes that side to slow down or stop while the other side continues at
the previous speed, causing the tank to turn towards the slower tread.
This sketch shows a wave ray striking an interface between old medium and new medium. A normal line has been
drawn as a dotted line perpendicular to the interface. The angle between the incident ray and the normal line is called
theangle of incidence, shown asθi, and the angle between the refracted ray and the normal line is called theangle
of refraction,θr.