wavelength of red light is about 700 nm, so using red light the difference in height between two fringes is half that, or 350 nm, about 1/100 the diameter of a human hair. Since the gap between the glasses increases radially from the center, the interference fringes form concentric rings. For glass surfaces that are not spherical, the fringes will not be rings but will have other shapes. Quantitative Relationships For illumination from above, with a dark center, the radius of the Nth bright ring is given by where N is the bright-ring number, R is the radius of curvature of the glass lens the light is passing through, and λ is the wavelength of the light. The above formula is also applicable for dark rings for the ring pattern obtained by transmitted light. Given the radial distance of a bright ring, r, and a radius of curvature of the lens, R, the air gap between the glass surfaces, t, is given to a good approximation by where the effect of viewing the pattern at an angle oblique to the incident rays is ignored. Thin-film interference The phenomenon of Newton's rings is explained on the same basis as thin-film interference, including effects such as "rainbows" seen in thin films of oil on water or in soap bubbles. The difference is that here the "thin film" is a thin layer of air. References Further reading External links Newton's Ring from Eric Weisstein's World of Physics Explanation of and expression for Newton's rings Newton's rings Video of a simple experiment with two lenses, and Newton's rings on mica observed. (On the website FizKapu.) Interference Optical phenomena