Young's Double Slit Spacing


In the Young's Double Slit Experiment, should the distance between the two opening of the slits be very close together? It is said so in our textbook but I don't understand why. If the distance is very small, compared to the point where the two rays intercept, then the phase difference will appear to be zero. So why's the point in doing this experiment?


In the double slit experiment the assumptions made in the analysis is that the distance between the slits is small compared to the distance from the slits to the screen, and that the distance between the slits is large relative to the wavelength of the light. For visible light the wavelength is a fraction of a micrometer. The distance between the slits is usually a fraction of a millimeter and the distance to the screen is about 1 meter. These conditions yield a satisfactory interference pattern.