Principle of Thin Film Interference
- 1. The volatility of light
In the 1860s, American physicist Maxwell developed the electromagnetic theory, pointing out that light is a kind of electromagnetic wave, which made the wave theory develop to a quite perfect stage.
From the wave-particle duality of light, we know that light, like radio waves, X-rays, and X-rays, are all electromagnetic waves, but their frequencies are different. The relationship among the electromagnetic wave’s wavelength λ, frequency u and propagation velocity V is:V=λu
Since electromagnetic waves of various frequencies propagate at the same speed in a vacuum, electromagnetic waves of different frequencies have different wavelengths. High-frequency wavelengths are short, and low-frequency wavelengths are long. To facilitate comparison, radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays can be arranged into a spectrum according to their wavelengths (or frequencies). This spectrum is called the electromagnetic spectrum.
In the electromagnetic spectrum, radio waves have the longest wavelengths, and radio waves are divided into long waves, medium waves, short waves, ultrashort waves, and microwaves due to different wavelengths. Followed by infrared light, visible light and ultraviolet light, these three parts are collectively called optical radiation. Among all electromagnetic waves, only visible light can be seen by the human eye. The wavelength of visible light is between 0.76 microns and 0.40 microns, which is only a small part of the electromagnetic spectrum. X-ray again. The electromagnetic wave with the shortest wavelength is the y-ray.
Since light is an electromagnetic wave, in the process of propagation, it should realize its characteristics-interference, diffraction, polarization and other phenomena.
- 2. Thin-film interference
The film can be a transparent solid, a liquid, or a thin layer of gas sandwiched between two pieces of glass. The incident light is reflected by the upper surface of the film to obtain the first light, the refracted light is reflected by the lower surface of the film, and then refracted by the upper surface to obtain the second light. The two lights are on the same side of the film and are separated by the same incident vibration. , Is coherent light, which belongs to sub-amplitude interference. If the light source is an extended light source (surface light source), interference can only be observed in the specific overlapping area of the two coherent beams, so it is localized interference. For flat films with two surfaces parallel to each other, the interference fringes are localized at infinity, usually by means of a convergent lens to observe in the image square focal plane; for wedge-shaped films, the interference fringes are localized near the film.
Both experiments and theories have proved that interference fringes can only be generated when two columns of light waves have a certain relationship. These relationships are called coherence conditions. The coherent conditions of the film include three points: the frequency of the two light waves is the same; the vibration direction of the light waves is the same; the phase difference of the two light waves remains constant.
The formula for the optical path difference of thin-film interference of two coherent light is: Δ=ntcos(α)±λ/2
Where n is the refractive index of the film; t is the film thickness at the incident point; α is the refraction angle in the film; λ/2 is due to the two coherent beams at two interfaces with different properties (one is the optically thin medium to the optically dense The other is the additional optical path difference caused by the reflection from the optically dense medium to the optically thin medium. The principle of thin-film interference is widely used in the inspection of optical surfaces, the precise measurement of tiny angles or lines, the preparation of anti-reflection films and interference filters, etc.
Light is radiated by changes in the motion state of atoms or molecules in the light source. The light waves emitted by each atom or molecule each time are only a short row, and the duration is about 1 billion seconds. For two independent light sources, The three conditions for interference, especially the condition that the phase is the same or the phase difference is constant, are not easy to meet, so two independent general light sources cannot constitute a coherent light source. Not only that, even the light emitted by different parts of the same light source, because they are emitted by different atoms or molecules, generally will not interfere.