The principle of thin film interference
1. The fluctuation of light
In the 1860s, American physicist Maxwell developed the theory of electromagnetism, stating that light is an electromagnetic wave, which led to the development of wave theory to a fairly perfect level.
From the wave particle duality of light, it can be inferred that light, like radio waves, X-rays, and X-rays, is an electromagnetic wave, albeit with a different frequency. The wavelength of electromagnetic wavesλ The relationship between frequency u and propagation rate V is:
V=λ Due to the equal propagation speed of electromagnetic waves of different frequencies in vacuum, the wavelengths of electromagnetic waves with different frequencies are also different. The wavelength with high frequency is short, and the wavelength with low frequency is long. For ease of comparison, they can be arranged in sequence according to the wavelength (or frequency) of radio waves, infrared rays, visible light, ultraviolet rays, X-rays, gamma rays, etc. This spectrum is called the electromagnetic spectrum.
In the electromagnetic spectrum, the longest wavelength is radio waves, which are further divided into long waves, medium waves, short waves, ultra short waves, and microwaves due to their different wavelengths. Next are infrared, visible light, and ultraviolet radiation, which are collectively referred to as optical radiation. Among all electromagnetic waves, only visible light can be seen by the human eye. The wavelength of visible light is approximately between 0.76 micrometers and 0.40 micrometers, accounting for only a small portion of the electromagnetic spectrum. Once again, it's X-rays. The shortest wavelength electromagnetic wave is gamma ray.
Since light is an electromagnetic wave, its characteristics such as interference, diffraction, polarization, etc. should be manifested during propagation.
2. Thin film interference
The film can be a transparent solid, liquid, or a thin layer of gas sandwiched between two pieces of glass. The incident light is reflected by the upper surface of the thin film to obtain the first beam of light, and the refracted light is reflected by the lower surface of the thin film and then refracted by the upper surface to obtain the second beam of light. These two beams of light are separated by the same incident vibration on the same side of the thin film and are coherent light, belonging to amplitude splitting interference. If the light source is an extended light source (surface light source), interference can only be observed in a specific overlap area between two coherent light beams, therefore it belongs to local interference. For a planar thin film with two parallel surfaces, the interference fringes are localized at infinity and are usually observed within its image focal plane using a converging lens; For wedge-shaped thin films, the interference fringes are localized near the film.
Both experiments and theory have proven that interference fringes can only be generated when two columns of light waves have a certain relationship, which is called the coherence condition. The coherence conditions of thin films include three points: the frequency of the two light waves is the same; The vibration direction of the beam of light waves is the same; The phase difference between two light waves remains constant.
The formula for the optical path difference of two-phase dry light in thin film interference is:
Δ =ntcos(α)± λ /In equation 2, n is the refractive index of the thin film; T is the thickness of the film at the incident point; α The refractive angle within the thin film; λ /2 is the additional optical path difference caused by the reflection of two coherent light beams at two interfaces with different properties (one from optically sparse medium to optically dense medium, and the other from optically dense medium to optically sparse medium). The principle of thin film interference is widely used in the inspection of optical surfaces, precise measurement of small angles or lines, preparation of anti reflection films and interference filters, etc.
Light is radiated by changes in the motion state of atoms or molecules in a light source. Each atom or molecule emits only a short column of light waves, with a duration of about 1 billion seconds. For two independent light sources, the three conditions for interference, especially the condition of the same phase or constant phase difference, are difficult to satisfy. Therefore, two independent general light sources cannot form a coherent light source. Moreover, even light emitted from different parts of the same light source, due to their different atoms or molecules, generally do not interfere