Diffraction & Huygen's Principle

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  • 0:02 What is Diffraction?
  • 0:50 Examples of Diffraction
  • 2:21 Huygen's Principle
  • 3:55 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to explain what diffraction is, provide some real-life examples of diffraction, state Huygen's Principle, and explain how Huygen's Principle applies to those real-life examples. A short quiz will follow.

What is Diffraction?

We live in a world full of waves: sound waves, water waves, light waves, microwaves... lots and lots of waves. So the better we understand them, the better we understand the world in general. Waves have certain behaviors and characteristics that are particularly distinctive: they reflect, they refract, but they also diffract.

Diffraction is what happens when a wave hits an obstacle or goes through a slit - it is the spreading out of the wave around an obstacle, or the spreading out of a wave as it goes through a slit. We can represent waves with a series of lines called wavefronts; these are the peaks of the waves. After diffraction, straight, parallel wavefronts will become curved. But, what does that mean for real life? How exactly does that affect us?

Wavefronts
wavefronts

Examples of Diffraction

Let's go back to the days when you were playing hide and seek with your friends. Maybe you still do. I know I would. But, let's also say that you're not very good at hiding and decide to hide behind a large tree. When the seeker shouts, 'Ready or not, here I come!' you can hear his voice quite well. But why?

It might seem like an odd question. Of course, you can hear his voice! But, that's only because it's what we're used to in our everyday lives. Imagine for a second that instead of a sound wave, light waves came out of the seeker's mouth... from behind a tree, you wouldn't hear it, because light travels in a straight line. Well, all waves, including sound waves, actually travel in a straight line. So why can you hear it?

It's all because of diffraction. Sound waves have a relatively large wavelength - they're really big waves. In addition, because of that, even large objects and slits cause them to diffract. So, the objects in a forest - trees, shrubs, unsuspecting kids, cause the sound waves to spread out around the gaps and reach you behind the tree. If it wasn't for diffraction, you would hardly hear anything at all.

Light has a much smaller wavelength, and because of this, a much smaller slit is needed for diffraction to happen. But, if you have a door with a ray of light coming through the tiny gap, and you make that gap small enough, you may see the ray spread out. And, if you're behind a couch, you'll find that it's not as pitch-black as you would expect.

Even water waves diffract. When we use a ripple tank in physics we see some of the same diffraction patterns that we see with lasers when we're studying light. Water waves will spread out around a barrier. So, a sea wall really does need to be complete to fully stop the waves getting through.

Huygen's Principle

When you shine a light through a tiny slit, a spreading out of the light isn't the only thing you see. If you shine the resulting light onto a screen, you might notice a weird pattern. This is called an interference pattern. Dark and light areas, one after another in a series of lines. Or, if it's a circular aperture, you'll see some concentric circles. It happens due to diffraction.

But, how can a wave interfere with itself? It might be easy to imagine two separate beams of light interfering with each other - when the peaks of the two waves hit the screen together (or the troughs for that matter), you'd get a light patch, and when a peak of one wave hits the screen with the trough of the other, you'd get a dark patch. That would explain the pattern.

But, why would a single beam of light create an interference pattern? The answer to that comes from a guy called Christian Huygens, a French physicist who proposed a wave theory of light many years before Maxwell took all the credit with his discoveries and equations. Hopefully, he wasn't too bitter about it.

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