Huygens’ PrincipleLearning Object

Christiaan Huygens

• A seventeenth-century Dutch mathematician and scientist (physicist, astronomer, probabilist, horologist)

• In 1678, Huygens proposed that every point on the wave front of a wave was a source of a spherical wave

• The resultant wave is determined by adding all the waves from the point sources

Christiaan Huygens

Consider a point source of sound

Step 1: Draw a spherical wave around the sound source

Let’s Draw!

Step 2: Draw a few equally spaced points (the blue dots) on the circular wave front

Continue Drawing…

Step 3: Draw arcs of circles around each point on the wave front

Continue Drawing…

Step 4: Draw a curve (the outer black circle) tangent to the individual circular wave fronts from the point sources (the blue arcs)

Can be created by dropping a straight stick horizontally into the pond

Drawing using Huygens’ principle (consider wave propagating to the right):

What about plane waves?

Incident wave front

3 wave fronts (black lines) are shown on the drawing on the right

We can continue drawing more wave fronts on the right

As the wave propagates, it remains as a plane wave (wave fronts are shown by black vertical lines)

Plane Waves

Direction of wave movement

Imagine the situation...

You’re at a show called “Who Wants to be a Millionaire”, and you’re presented with this million dollar question:

What does the shape of the wave fronts look like after it pass through the slit?

Question Time!

slit

Note: black lines represent crest of plane wave

You’re presented with three choices:

BA

C

At this point you might panic :O

But if you know Huygens’ Principle, there’s nothing to be afraid of!! :D

The wave propagates to the right

The wave eventually reaches a slit that is wider than the its wavelength

Note that…

slit

Start drawing wave fronts using Huygens’ Principle!

And you’ll get…

The red lines represent the newly drawn wave fronts

This is what the propagation of the wave looks like as the wave passes through a slit that is wider than its wavelength

More clearly…

The answer is B

In the previous example, we have successfully used Huygens’ principle to show what a diffraction of wave through a slit would look like!

It shows how the shape of the wave fronts is slightly curved after passing through the slit.

Wrapping up…

Thanks for watching!!

Hawks, R., Iqbal, J., Mansour, F., Milner-Bolotin, M., & Williams P. 2014. Physics for Scientists and Engineers: An Interactive Approach (1st ed.) Nelson.

Christiaan Huygens. Wikipedia. Retrieved from http://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle [accessed 12 Mar 2015]

Jones, A. Z. Huygens’ Principle. About Education. Retrieved from http://physics.about.com/od/mathematicsofwaves/a/huygensprincipl.htm [accessed 12 Mar 2015]

Citations