physics learning object 7 - huygens principle

1. Huygens’ Principle
Understanding and using Huygens’
principle with wave propagation
- Zeanna Janmohamed

2. overview
• The principle states that every point on a wave
front is the source of a wavelet
– These wavelets will spread out in a forward
direction at the same speed as the inception wave
– The new wave front will be a line tangent to the
wavelet
Figure 1.0 wavelets in water
http://thumbs.dreamstime.co
m/t/drop-water-4865181.jpg

3. History and overview
• Christian Huygens was a 17th century Dutch
scientist
– In 1678 he proposed that the sum of secondary
waves which are the result of disturbance
determines the form of the new wave
– He derived the laws of reflection and refraction
but could not explain diffraction
• This was later added to by Augustin-Jean Fresnel

4. History and overview cont.
• Recognized that the
propagation of a wave could
be described by considering a
point on a wave
• Result wave can be
determined by adding all the
values from the point sources
Figure 1.1 straight wave front: each point emits
a semicircular wavelet
https://www.boundless.com/physics/textbooks/boundless-physics
-textbook/wave-optics-26/diffraction-175/huygens-principle-636-5593/

5. How to use Huygens’ principle
1. Draw a spherical wave Select equal speed
points around the wave
2. Draw circles from different points along the
wave that have centers at different points
along the initial wave front This way the
outer edges of the circles will overlap slightly
3. Draw tangent lines to the individual circular
wave front

6. Huygens’ principle – why is it
important
• Huygens’ principle of diffraction can be related to
the equation d=vt (where d=distance, v =
propagation speed, and s=time)
• This means that each wave fronts emits their own
v
– Emitted waves are semi circular as shown from the
previous side
– Principle works for all waves and can describe
interference and refraction

7. examples
• Imagine walking beside someones room in residence,
they are watching a movie and their door is closed. Can
you hear it too well?
– Probably not
• What happens when the person’s door is open though?
– You can hear it not only in front of their room but along
the hall too!
• This is a direct result of diffraction, because waves
bend in the tangent lines. Similarly light does the same
thing and can be used to understand the basic concept
of diffraction

8. Plane waves
• Plane waves have wave front that lie in planes
that are parralel to one another
Figure 1.2 plane wave anatomy
http://commons.wikimedia.org/wiki/File:Plane_Wave_Oblique_View.jpg

9. Examples of plane waves
• This is what we see in waves themselves
– Once created, a plane wave can continue to
progragate as a plane wave
• If the wave extends in a horizontal direction the viritcal
distance shows the amplitude of the wave
Figure 1.4 wave
fronts
http://www.ualberta
.ca/~pogosyan/teach
ing/PHYS_130/image
s/500px-
Plane_wave_wavefro
nts_3D.svg.png

10. Test your understanding
• Question: using Huygens’ princple explain the
diffraction of waves in water in a ripple tank as
these waves pass through a slit in the barrier.
– See next slide for answer
Figure 1.4 a ripple tank set up
http://upload.wikimedia.org/
wikipedia/commons/thumb/a
/af/Simple_ripple_tank.svg/2
000px-
Simple_ripple_tank.svg.png

11. Answer to testing
• Because each point on the initial wave front
plane that enters the slit will act as a source of
a secondary wavelet, the wavelets will
disperse in all directions after the slit causing
the waves to spread into regions beyond those
in line with the given slit.