I have some questions regarding Young's double slit experiment. 1.The first concerns the no. of possible fringes that will be formed. Sample question is as: Slit Separation = 1mm and wavelength of light is 5250 A. No. of illuminated fringes=? The basic problem is that I don't understand why the no. of fringes is not infinite as there is no limitation on the extent of the screen and light can interfere anywhere. 2.Then there is this thing about the incident beam being at an angle to the normal of the plane of the slits. Does it really matter if the light makes an angle? Anyhow the points illuminated are going to behave as secondary sources. Although I think it will but i can not put my thoughts together. Any help would be appreciated.