The device for Newton's rings is an interferential apparatus with two beams by amplitude splitting and consists of a planoconvex lens having a large radius of curvature lying with its spherical face on the flat surface of a glass disk.
On illuminating the device from above with a parallel beam of monochromatic light, it is possible to see a system of alternating light and dark rings, known as Newton's rings, reflected on the surface of the lens. They are generated by interference in the thin layer of air of varying thickness between the lens and the flat surface it is lying on; they all have their centre at the point of contact. The ring at the centre is dark and the others become closer and closer together the farther they are from the centre.
If the device is illuminated by white light the rings are coloured.
The rings are thus of equal thickness and are circular due to the cylindrical symmetry of the device. ...continues