in newtons experiment how we detect lens has some defect

The wavelengths will be shorter which means the rings will be more closely spaced. The center spot should continue to be dark since both air and water have indices of refraction less than that of glass. Thus the reflection from the bottom of the lens will not invert in phase and that from the bottom of the water will, creating a net phase reversal and cancellation at the center where the path length is zero, just as with air.