This article introduces the topic of diffraction—lights curious capability to bend around corners.

Previous articles described light in three different ways depending on the circumstances:  

What does it mean for light to behave like a wave?

As in the previous post, it is useful to consider the analogy of waves in water. Notice that waves do not necessarily travel in a straight line—they tend to spread out as they ripple forward. The attached video shows waves from a stream passing through a small break in the shoreline. Notice that the waves, which start out moving from left to right, begin to spread out again when they get past the barrier. In effect, the waves have bent around the edges of the opening.

The phenomenon of light bending around an edge is called diffraction.  The amount that a light wave diffracts depends on two things, the wavelength of the light and the relative size of the hole. Remember that the wavelength of light corresponds to its color.

There are two facts to remember about light diffraction:

  • Narrow openings cause more bending than wider openings.
  • Light with a longer wavelength bend more than light with a shorter wavelength.

The three drawings above illustrate light diffraction for two different colors of light passing through two different size holes. In the first drawing, red light diffracts outward as it passes through an opening. In the second drawing, light of the same red wavelength bends more when it passes through a narrower opening. In the third drawing, shorter-wavelength blue light diffracts less than longer-wavelength red light when it passes through the same size narrow opening.

Collecting this information, the “very least you need to know” definition of light diffraction is:

Diffraction refers to light’s ability to bend around corners, with longer wavelengths bending more than shorter wavelengths, and narrower openings causing more bending than wider openings.

Why does light diffraction matter?

A prominent example of light diffraction occurs within camera lenses. Because of diffraction, a camera can never bring an image to a perfect focus. This is because incoming light will always bend a small amount every time it travels past the edges of a component within the camera. That inescapable bending means that all lenses have a diffraction limit. A lens that performs as close to perfect as possible is called “diffraction limited.” A diffraction-limited lens focuses light to the smallest point that physics allows.

Re-examining the video, notice that, aside from bending around corners, the waves interact with one another to create complicated patterns across the surface of the water? The ability of waves to interact with one another is called interference, and it is closely linked to diffraction. Interference is the subject of the next article.

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