Final answer:
The single slit in a double slit experiment must be narrow in order to allow light to interact with an object comparable in size to its wavelength, creating observable diffraction patterns.
Step-by-step explanation:
The reason the single slit in a double slit experiment must be narrow is because light needs to interact with an object that is comparable in size to its wavelength to show significant wave effects, such as single-slit diffraction patterns. When a slit is a few times greater than the wavelength of light, it allows light waves to interfere in a way that creates observable diffraction patterns. These patterns are a result of each point along the slit acting as a point source of light, as described by Huygens's principle.
If the slit were wider, each point across the width of the slit would act as a separate point source, causing the light to spread out more and creating more complex diffraction patterns that could overwhelm the simpler double-slit interference patterns. The precise control of slit width is crucial for observing the characteristic interference fringes. In addition, when studying other properties such as single-slit interference, the specifics of slit dimensions and their relation to the wavelength become even more important.