Theory
There is light incident on the flat plane of the convex lens that is situated on the optically flat glass surface below, the light passes through the glass lens until it comes to the glass-air boundary, and here the light goes from a higher refractive index (n) value to a lower n value. The light passes through this boundary and suffers no phase change. Also at this boundary, some light is transmitted into the air, and some light is reflected. The light that is transmitted to the air travels a distance, t, before it is reflected at the flat surface below; the air-glass boundary causes a half-cycle phase shift because the air has a lower refractive index than the glass. The two reflected rays now travel in the same direction to be detected. The convex lens touches the flat surface below, and from this point, as one gets farther away, the distance t increases, because the lens is curving away from the surface:
- so therefore:
- and finally, we have:
Read more about this topic: Newton's Rings
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