问答题

As shown in Figure, an air wedge is formed between a circular plane-concave lens and a glass plate. The monochromatic light with wavelength λ is normally incident on it and the interference fringes of the reflected light can be seen. The maximum thickness of the air wedge is equal to d =7λ/4. The number of the dark fringes what can be seen is            . (if possible also considering edge fringe)