What it shows:
Waves reflecting from two surfaces can interfere constructively and destructively. In this case it is light waves that are being reflected at glass/air and air/glass interfaces. The interference produces a concentric ring pattern of rainbow colors in white light, or dark and light rings in monochromatic light.
How it works:
The convex surface of a long focal length lens (large radius of curvature) is placed in contact with a plane glass disk and clamped together, as shown in cross section below. 1 Adjustment screws (not shown) are tightened to secure intimate contact at the center.
A thin film of air (greatly exaggerated in the illustration) is formed between the two surfaces of glass. The ensemble is viewed under reflected light from an extended light source. We ignore reflections from the top of the plano-convex lens and the bottom of the plane glass disk - these reflections just contribute to the overall glare. The reflections of interest are those involving the surfaces in contact. There is no phase change at the glass-air surface of the convex lens (because the wave is going from a higher to a lower refractive index medium) whereas the reflection at the air-glass surface of the plane disk suffers a half-cycle phase shift. With R as the radius of curvature of the convex lens, the relation between the distance x (distance from center) and the "air-film" thickness d is given by 2
x2 = 2Rd
and the radius of the mth bright ring is given by
xm = [(m + ½)λR]½
The center of the pattern is black. Here the two glass surfaces are in intimate contact and there is no reflection because it is as if there were no surface. 3 The first maximum of reflected light is almost white in color. This is because the distance between the two glass surfaces is such that it's almost (1/4) λ for the entire spectrum. 4 Successive rings exhibit more and more color. The most monochromatic ring will be the one where the thickness is some odd number N of (1/4) λ for green, and where blue is about (N+1)(1/4) λ and red is (N-1)(1/4) λ. This puts blue and red at reflection minima while green is at a reflection maximum.
Setting it up:
The entire setup fits very nicely on one of the lecture carts and can be readied beforehand and wheeled out complete. For a uniform white light source, a light box 5 is used. For monochromatic illumination, use the green mercury light box. 6 Place a black felt cloth under the Newton's Rings apparatus and position the color CCD video camera and light source on opposite sides of the apparatus to secure an approximate 45° reflection. The 50 mm Nikon lens with a 5 mm extension ring will nicely frame the small apparatus. The 35" Mitsubishi monitor will give the truest colors; the rear-projection video is pretty good and doesn't use up floor space.
A classic demonstration dating way back to none other than Newton himself. However, his explanation of the rings (light particles undergoing "fits and starts" of easy and difficult reflections) left something to be desired.
1 The metal frame with its three adjusting screws (not shown) is 55 mm in diameter and the aperture is 26 mm in diameter. Our apparatus is an old Welch Scientific cat. no. 3552; most suppliers of instructional scientific apparatus sell some version of this.
2 E. Hecht and A. Zajac, Optics, (Addison-Wesley, Reading MA, 1974) pp 299-300
3 Alternatively, one can argue that the two reflections from the two surfaces cancel each other out because the second reflection (when light travels from air to glass) suffers a 180° phase shift.
4 For example, let's assume that the distance is exactly (1/4) λ for green (the center of the spectrum at 550 nm). The distance is thus 138 nm. This is 0.3 λ for blue (assuming 450 nm) and 0.21 λ for red (at 650 nm), both numbers being close to 0.25λ.
5 Logan model 2020 PortaView Light Box slide/transparency viewer; it measures approximately 45×35 cm.
6 Unilamp model UL-12, manufactured by Midwest Scientific Co., Sequal Manufacturing Inc., 3015 Power Drive, Kansas City, Kansas 66106; (913) 236-9674