Diffraction Dilemmas!

Diffraction is a strange phenomenon that occurs any time light interacts with a opaque boundary. In the picture to the left, ocean waves that encounter the gap between the peninsula and the islet are diffracted into the bay. You can see the straight waves approaching from the bottom of the picture and the waves becoming curved after they pass through the gap. This behaviour is easy to see when the wavelength of the wave are so large as to be visible, but what about when they are so small that you cannot see the individual waves, like in light?

Well, instead of looking for the waves themselves, we look for the effects of the diffraction. As you can see in the above picture, diffraction causes waves to to bend around an obstacle. When light waves interact with a boundary, they too will bend. You can see this when you turn on a single light (a point source, such as an LED, or stretching the definition slightly, a standard incandescent lightbulb), and look at the shadows produced. These shadows aren’t simply black and white, the edge of each shadow grades from white to gray to black. The region of gray-ness is where light is diffracting past an edge. This effect is strongest where the edge of the object is sharp and clearly defined. The razor blade shadow above shows multiple diffraction shadow edges – but a better question might be why are there bright lines as well as shadows?

This picture shows a straight wave approaching a gap that is four times as wide as the the wavelength of the wave (click to enlarge). The wave passing through this gap acts like a sequence of point sources of light (to explain this, imagine each point source as a the source of a huyghen’s wavelet). From our previous studies of superposition and wave mechanics, we know that if two waves interact, they may interfere constructively (creating a higher peak amplitude/ intensity) or destructively (creating a minimum height section). The distance from each point source to any point in the pattern is different, and if measured in wavelengths of the diffracted waves can be described as a difference in path length, or path difference. If this difference is whole number of wavelengths, constructive interference will occur, creating a bright point (or maxima). If the difference is odd number of half wavelengths (e.g. 1 λ/2, 3 λ/2, 5 λ/2 etc), then destructive interference will take place, creating a darker point (or minima). Here are two links (1, 2) that allow you to explore the effects of changing wavelength and gap size on the amount of diffraction occuring (and the number of maxima and minima produced). It doesn’t matter if the wave is light (as above, or sound or any other type) waves will diffract around barrier or through gaps of the appropriate dimensions (determined by comparing wavelength to gap width (why: as the gap is wider, more point sources can be accomodated. As you gain more point sources, the overall pattern becomes more even – to any one point, multiple point sources may create constructive or destructive interference – thus removing the clear distinction of specific minima and maxima)).

Ever wondered why there is rainbow effect while looking at a CD (or a oil slick on the surface of water)? It’s all about diffraction, baby!

See you in class!

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