The width of a rectangular slit is measured in the laboratory by means of its diffraction...

The width of a rectangular slit is measured in the laboratory by means of its diffraction pattern at a distance of 2 m from the slit. When illuminated normally with a parallel beam of laser light (632.8 nm), the distance between the third minima on either side of the principal maximum is measured. An average of several tries gives 5.625 cm.

a. Assuming Fraunhofer diffraction, what is the slit width?

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b. Is the assumption of far-field diffraction justified in this case? What is the ratio L/Lmin?

Figure 1.