Single Axis Diffraction Grating

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When we begin learning about light, we usually start by talking about the colors of the spectrum and the fact that white light can be broken up, or dispersed into a spectrum of colors. To disperse light into its spectrum, Sir Isaac Newton used a prism. However, in recent years the diffraction grating has replaced the prism for this purpose because it is easier, more effective, and less expensive.

Diffraction gratings are not new. They've been the basis of spectroscopic instruments for a long time, but these instruments are not necessary for many learning purposes. You can see the exciting and detailed spectra simply by holding a diffraction grating up to your eye and looking through it at a light source in a dark place.

Eventually, the question arises, 'How does a diffraction grating work?' It's not easy to find an answer to this question that doesn't get mathematical, yet explains the principal in a satisfying way. The following attempts to do that.

Waves in a Pond
Light waves are similar to water waves in many respects. Let's start with the familiar situation of water wave ripples due to a dropped pebble. Their spread (concentric circles) can be understood by considering each point along a wave, or wave front, to be the source of a new wavelet, each source having the same phase.

Now let's apply this to a wave that encounters an obstruction such as a narrow slit. The wave spreads out in a circular pattern. The difference between successive peaks or valleys is called the wavelength.

Now let's increase the number of narrow slits, equally spaced. This is called a diffraction grating.

When a light wave encounters a diffraction grating, the light spreads as if it originated from many point sources, each in phase with one another. Each wave spreads out in a circle, but now there are centers at each slit. If one wavelet's peak lies on another wavelet's valley, the result is neither peak nor valley, but rather cancellation. However, if one wavelet's peak lies on another wavelet's peak, they add constructively, making a wave twice as high.

There are special directions where cancellation is avoided and the wavelengths add constructively. The direction is different for different colors because different colors have different wavelengths. For example, since the wavelength of red light is longer than the wavelength of blue light, a red beam is diffracted or bent further than a blue beam when it passes through the diffraction grating.

This is how a diffraction grating breaks up the colors of white light. White light has many colors. Between red and blue are other colors, like orange, yellow, and green, whose wavelengths are intermediate between those of red and blue light.

Putting all this together, we can see how a beam of white light, which contains all the colors, gets diffracted or bent into a spectrum of colors.