Final answer:
As the wavelength of light increases, the spacing between the interference fringes observed by a photodetector becomes wider because the fringe separation is proportional to the wavelength.
Step-by-step explanation:
When the wavelength of light is slowly increased, the fringes observed by a photodetector in an interference pattern will change. The distance between the fringes will increase because the fringe separation is directly proportional to the wavelength. As the wavelength increases, each point on the interference pattern moves to a position farther from the central maximum, resulting in wider spacing between the fringes. Additionally, if the slit separation is much larger than the wavelength, an interference pattern with a large number of fringes may not be observable, and instead, two bright lines may appear, in accordance with Rayleigh's criterion when light behaves more like rays.