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Huygens' principle states that all points on a wave front serve as point sources for secondary spherical waves that propagate outward. The position of the wave front at some later time is determined by the tangent to the surface of the secondary wave fronts. Huygens' principle can be used to predict observed optical phenomena such as refraction. Although the principle may seem strange and contrived, it is a direct consequence of the differential wave equation. This Illustration shows you Huygens' principle applied to light passing between two mediums. Restart.
The animation begins with the n1 = n2 animation. Click "play" to begin. You will see a wave front, represented by a white line, moving at an angle across the screen. Huygens' principle applies to all points along the path of the wave front. However, to make it simple, the visualization of the creation of the secondary wave fronts is only shown for the points down the center of the applet. In this case the medium on the right and left of the points is the same. As the applet plays, carefully watch as the secondary wave fronts are formed at the points in the center. Notice how the wave front, now defined as the tangent to the surface of the secondary wave fronts, is exactly the same as it was before. View the n1 = n2 animation several times until you feel comfortable with what it represents.
Now initialize and play the n2 > n1 animation. In this animation, the wave front passes from one medium to another. Because n2 > n1, the waves slow down in the second medium. Carefully watch as the wave front passes from one medium to another. Since the wave fronts are traveling slower in the second medium, you see the primary wave front bend downward. This is particularly apparent if you pause the applet just as the wave front reaches the medium and then step forward as the wave front passes into the new medium.
Finally, initialize and play the n2 < n1 animation. In this case the waves speed up in the second medium and you see the wave front bend upward.
Illustration authored by Melissa Dancy.
Script authored by Morton Brydensholt.
© 2004 by Prentice-Hall, Inc. A Pearson Company