Christiaan Huygens was a 17th century Dutch scientist who made important contributions to physics, including developing Huygens' Principle which describes wave propagation. Huygens' Principle represents each point on a wave front as a point source of secondary spherical wavelets, with the new wave front being the tangent lines to these secondary wavelets. Plane waves have parallel wave fronts, while spherical waves have expanding spherical wave fronts due to the point source nature of the secondary wavelets.

1. Huygens’ Principle
Physics 101 Learning Objects
Stacey Young

2. Who is Christiaan Huygens?
Christiaan Huygens’ was born in 1629 into a
Dutch family. In 1645, he went to University of
Leiden to study law and mathematics. During his
studies, he corresponded with other geniuses,
such as Mersenne, who compared Huygens’
intelligence to the one of Archimedes’.
So why is he important?
Huygens’ discoveries after his studies and
throughout his lifetime has further developed
what we learn in Physics today, including the
formula for deriving the period of a pendulum and
of course, Huygens’ Principle!

3. Huygens’ Principle
This principle describes a wave’s propagation using this procedure.
1. Make some points on a wave front. Each point should be equally spaced from
each other and equal distance from the original wave front
2. Draw a circle around one point. Now draw the same sized circle around each
point you have marked previously.
3. Draw tangent lines to the circle and there you have your new wave.

4. Wave Propagation
Plane Waves vs. Spherical Waves

5. Plane Waves
 Each new wave front of a plane wave is
parallel to the previous wave. Imagine
you dropped a pen into water
horizontally. To determine the resulting
wave using Huygens’ Principle, the points
we draw are all on that linear horizontal
line. The resulting wave will be linear as
well.

6. Spherical Waves
 Each new wave front of a spherical
wave is going to be spherical, only
with a bigger radius than the
original wave front. Instead of
dropping a pen into the water, we
dropped a baseball. To determine
the resulting wave the points drawn
on the original wave makes a
circle. The tangents created by
those points will create our
resulting circle.

7. Checking for Understanding
 Using Huygens’ Principle, find the resulting wave front.