2. H 2 is neglected in the derivation, which also makes the measurement result small. This factor has little influence.
Thirdly, in the experimental operation, because it is impossible for the center to reach point contact, under the gravity and screw pressure, the lens will deform and the center will form dark spots, which will lead to the deviation of the measurement results.
In our formula, the difference between two levels is used, but this can only avoid the problem of determining the dark ring level, and can not really completely eliminate the influence of the size of the central dark spot on the results. Because the size of the central dark spot reflects the size of the lens deformation, not only the central deformation, but also the whole curved surface is deformed by the pressure and gravity of the screw. The larger the deformation is, the smaller the δh is, so the denominator (m-n) in the relation is different from that without deformation, but smaller, and it can be inferred that the measurement result is larger. The formula in the experimental book implies that only the center is flattened, and the deformation of other parts of the lens surface is not considered. In fact, when the lens is deformed, it is no longer spherical, and it does not strictly satisfy the relationship: Δ r 2 = 2R Δ h.
That is to say, under the same radius r, the corresponding air layer thickness h decreases, and the farther away, the smaller δ h, the smaller m-n and the greater the measurement result. This factor is the most influential one. The larger the central dark spot is, the more inaccurate and larger the measurement result is.
For this factor, there is a small paper entitled "The influence of the size of Newton's ring dark spots on the measurement results".