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Section 51.1 The Diffraction Phenomenon

Imagine shining a monochromatic light through a narrow hole of width a little larger than the wavelength. Rather than a simple shadow behind the hole, you will find that there is an interference pattern even though there is aparently only one source of light. This is illustrated in the diagram given in Figure 51.1.1. How's that possible? The wavefront emerging behind the hole must consist of many waves for them to produce interference pattern.

Figure 51.1.1. Illustration of diffraction from a single slit.

Similarly, if you place an opaque disk in the path of light, you normally see a shadow of the object behind it. But, if the disk is small enough, you will notice a bright spot in the dark shadow, as if light went around the disk.

The two examples above illustrate an innate property of all waves, not just light waves, of spreading out from a source and going around obstacles. This property is alled diffraction. The explanation of diffraction dates back to 1819, when Augustin Fresnel proposed his theory of diffraction in a winning essay to the French academy.

Soon after Fresnel's publication, S. D. Poisson, a skeptic of the theory, pointed out that Fresnel theory predicted a strange phenomeon that, if you place a disk in the path of the wave, the waves will bend around at the edge and then interfere constructively at the center of the shadow producing a bright spot, as illustrated in Figure 51.1.2. The spot is now called Poisson spot.

Figure 51.1.2. The shadow with Poisson spot and diffraction pattern.

Poisson put up monetary reward for anyone disproving this strange prediction. However, experiment validated the prediction and lent invaluable support to the wave theory of light.