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Young’s Double Slit Experiment

Exploring Huygens’ PrincipleLearning Object by Keila Stark, lecture section

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When I first attempted this week’s pre-reading, I had a difficult time visualizing the concepts behind Huygens’ Principle, and understanding what its ramifications are in the context of wave propagation. I decided to do extra reading online to force myself to understand the concept, as well as perform a very simple experiment to solidify my understanding.

Rationale

Background

Isaac Newton (1643-1727)“Light consists of particles”

Christiaan Huygens (1629-1695)“Light consists of waves” Both were able to support their hypotheses based on different aspects of light behaviour; while Newton was able to determine that light is comprised of many small particles through his colourful prism experiment, Huygens was better able to explain the reflection and refraction properties of light…

Background

Thomas Young (1773-1829)Proved Huygens’ idea that light behaves a wave with his double slit interference experiments.

Image source: http://creationrevolution.com/wp-content/uploads/2012/04/4-16-12-CREV-Sir-Thomas-Young-Md-Frs.jpg

Every point in the wave front of a travelling wave is the source of a spherical wave

The tangent to these spheres gives a new wave front

Huygens’ Principle

So what does this mean?

Imagine that the parallel black lines are wave fronts in a wave that is propagating to the right as indicated by the red arrow

Important note: a wave front is a collection of points that are in the same phase

Drawn is a collection of arbitrary points located on the wave front of equal distance…

The principle states that “every point on the wave front is a source of a spherical wave”. Here I’ve drawn the spherical waves corresponding to the arbitrary points from earlier, but in reality there is an infinite number of them.

New wave front

Here, the tangent line shared by all the spherical waves is drawn to the right of the original wave front, suggesting that, for a wave propagating to the right, a future wave front looks like that.

Also note that I have drawn in more spherical waves in green, showing that though we’ve depicted more spherical waves, the tangent line is still the same.

But that all seems rather simple… Couldn’t we have predicted what the new wave front looks like without drawing a bunch of fancy circles…?

Huygens’ idea of spherical waves helps us to understand and visualize wave behaviour when it propagates in a more complicated manner, for instance through tight spaces and around corners.

To demonstrate this idea, I decided to replicate Young’s double slit interference experiment.

I constructed an incredibly high-tech apparatus out cardboard, duct tape, and phone flashlight…

Light source Slits

Note the circles are not perfectly drawn, however the concept is still illustrated; when spherical waves of equal radii are drawn to surround points along the wave fronts, we can obtain the shape of resultant wave fronts.

Note also that when we complete the wave fronts, we get regions of constuctive (in pink) and destructive (in green) interference, which is why we see regions of higher and lower light intensity.