Final answer:
We assume the slits in two slit interference are point sources to ignore the effects of single slit diffraction, which is reasonable when the slit width is much smaller than the light's wavelength.
Step-by-step explanation:
When considering two slit interference, the assumption that allows us to ignore the effects of single slit diffraction is that the slits are very narrow, effectively acting as point sources.
Therefore, the spreading of light due to diffraction at each slit is minimal, allowing a clear pattern of interference between the two slits to be observed on the screen. This assumption is valid when the width of the slits is much smaller than the wavelength of the light being used, resulting in a scenario where the diffraction effects are overshadowed by the interference pattern produced by the two slits.