What Are Newton's Rings?

Newton's ring, also known as "Newton's ring". Optically, the Newton's ring is a thin film interference phenomenon. An interference pattern of light is some concentric circles of light and dark. For example, if the convex surface of a convex lens with a large radius of curvature is in contact with a flat glass, it can be seen that the contact point is a dark point in the sunlight or white light, and the surrounding is a circle of light and dark colored circles; When colored light is irradiated, it appears as some light and dark monochrome circles. The distances of these circles vary, and they gradually narrow as the distance from the center point increases. They are interference fringes formed by the light rays reflected on the sphere and the plane interfering with each other.

Newton ring

The Newton's ring experiment is like this: Take two glass bodies, one is a plano-convex lens for a 14-foot telescope, and the other is a large lenticular lens for a 50-foot telescope. Put a plano-convex lens on the lenticular lens so that its plane is downward. When the glass bodies are pressed against each other, various colors will appear around the contact point to form a color ring. Then these colors disappeared one after another in the center of the ring. When the vitreous body is compacted, the color that appears at the center of other colors at first appears to be an almost uniform color circle from the periphery to the center. When the vitreous body is compacted again, the color circle will gradually widen until New colors appear in its center. Continue this way, the third, fourth, fifth and other colors continue to appear in the center, and become a set of color rings wrapped outside the innermost color, the last color is a black dot. Conversely, if the upper glass body is lifted away from the lens below, the diameter of the color ring will become smaller, and its peripheral width will increase until its color reaches the center one after another, and then their width becomes quite large, which is smaller than It was easier to recognize and discern their colors before.

Newton measured the radii of six rings (measured at their brightest parts) and found such a gauge

Newton's ring phenomenon

Law: The square value of the radius of the bright ring is an arithmetic series consisting of odd numbers, namely 1, 3, 5, 7, 9, 11, and the square value of the radius of the dark ring is an arithmetic series consisting of even numbers, namely 2 , 4, 6, 8, 10, 12. For example, in the cross section of the convex lens and the flat glass near the contact point, the distance on the horizontal axis is plotted as an integer square root: 1 = 12 = 1.41, 3 = 1.73, 4 = 2, 5 = 2.24 and so on. At these distances, Newton observed maxima and minima of alternating light. As can be seen from the figure, the vertical distance between the two glasses is increased by a simple arithmetic progression, 1, 2, 3, 4, 5, 6, ... In this way, after knowing the radius of the convex lens, it is easy to calculate the thickness of the air layer at the dark ring and the bright ring. Newton measured it at this time: the darkest part of the first dark ring obtained by the light incident vertically The thickness of the air layer is 1 / 189,000 inches. Multiplying this half by the number of stages 1, 3, 5, 7, 9, and 11 gives the air layer thickness of the brightest part of all bright rings, which is 1 / 178000, 3/178000, 5/178000, 7/178000 ... their arithmetic mean 2/178000, 4/178000, 6/178000 ... etc. are the thickness of the air layer in the darkest part of the dark ring.

The radius of the interference dark ring generated by the Newton ring device is (kR), where k = 0,1,2 ...

Newton also replaced the air with water, and observed that the radius of the color circle would decrease. He not only observed the interference fringes of white light, but also the interference fringes between bright light, which was presented by monochromatic light.

Newton's ring devices are often used to verify the accuracy of the surface of optical elements. If the pressure between the convex lens and the flat glass is changed, the thickness of the air film between them can be changed slightly, and the stripes will move. With this principle, small changes in pressure or length can be accurately measured.

Theoretically, Newton's ring is one of the best proofs of the volatility of light, but Newton's is not true.

Newton's ring experimental instrument

From the international perspective, he explained the formation of Newton's ring from the particle theory he believed in. He believes that light is a stream of particles moving at high speed through a puppet, so in order to explain the appearance of Newton's rings, he put forward a complex theory of "a burst of easy reflection and a burst of easy transmission". According to this theory, he believes that "Every light enters a certain transient state when it passes through any refracting surface, and this state recovers at regular intervals during the light's advancement, and tends to each time it recovers. In order to make the light easily pass through the next refracting surface, between the two restorations, it is easy to be reflected by the next refracting surface. "He also called the distance between each return and the next return as" burst Interval. " In fact, what Newton refers to as the "interval between bursts" is the "wavelength" in the wave. Why is this so? Newton said vaguely: "As for what effect or tendency it is, it is the circular motion or vibration of light, or the circular motion or vibration of medium or something, and I will not discuss it here."

The Newton's ring meter is composed of a plano-convex lens L and a glass flat plate P with a radius of curvature R stacked in a metal frame F, as shown in the right figure below. There are three screws H on the side of the frame to adjust the contact between L and P to change the shape and position of the interference fringe. When adjusting H, do not overtighten the screws to prevent the glass lens from cracking and breaking due to excessive contact pressure. The picture on the right is the physical picture of Newton's ring .

Newton's ring argument

Although Newton discovered the Newton's ring and made accurate quantitative measurements, he could

Newton's ring

It is said that it has reached the edge of the wave theory of light, but due to his preference for the particle theory, it is still impossible to correctly explain this phenomenon. In fact, this experiment can be one of the strong evidence of the theory of light fluctuations. It wasn't until the early 19th century that British scientist Thomas Young satisfactorily explained the Newton's ring experiment with a wave theory of light.

Newton's ring application

Determine the convexity and concavity of the lens surface, accurately inspect the surface quality of the optical element, measure the curvature radius of the lens surface and the refractive index of the liquid.

When processing optical components, the principle of Newton's rings is widely used to check the accuracy of surface shapes of flat or curved surfaces.

It is used in spectrometers to separate composite light into monochromatic light.