Interference generated by thin films. A thin film can be a transparent solid, liquid, or a thin layer of gas sandwiched between two pieces of glass. The first beam of light is obtained after the incident light is reflected on the surface of the thin film, and the refracted light is reflected on the lower surface of the thin film, and then refracted on the upper surface to obtain the second beam of light. These two beams of light are on the same side of the thin film, separated by the same incident vibration, and are coherent light, belonging to amplitude division interference. If the light source is an extended light source (surface light source), interference can only be observed in a specific overlapping area of two coherent beams, so it belongs to localized interference. For a planar thin film with two parallel surfaces, the interference fringes are localized at infinity and are usually observed within its image square focal plane using a converging lens; For wedge-shaped thin films, the interference fringes are localized near the film.
The formula for the optical path difference of two-phase dry light in thin film interference is
In the formula, n is the refractive index of the thin film; T is the thickness of the thin film at the incident point; θ T is the refractive angle inside the thin film; ± λ/ 2 is the additional optical path difference caused by the reflection of two coherent beams of light at two interfaces with different properties (one is the optical sparse optical dense interface, and the other is the optical dense optical sparse interface). 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.
Equal tilt interference and equal thickness interference are two typical forms of thin film interference.
Interference caused by the reflection (or refraction) of light beams from the upper and lower surfaces of a thin film. A thin film is usually formed by a very thin transparent medium, such as a soap bubble film, an oil film on the water surface, an air film sandwiched between two pieces of glass, and a dielectric film coated on a camera lens. There are two simpler types of thin film interference, one is called equal thickness interference, which is caused by parallel light incident on a thin film with uniform thickness variation and refractive index Interference fringes formed on the lower surface. The same interference fringes are formed in areas with the same thickness of the film, so they are called equal thickness interference. Newton's ring and wedge plate interference belong to equal thickness interference. Another type is called equal inclination interference. When light with different inclinations is incident on a film with uniform refractive index and parallel upper and lower surfaces, light with the same inclination angle reflects (or refracts) on the upper and lower surfaces and meets to form the same interference fringes, Different interference fringes or dark fringes correspond to different inclinations, which are called equal tilt interference. Equal tilt interference generally uses an extended light source and is observed through a lens
Press two clean glass sheets tightly together, and the air layer between the two sheets forms an air film. Using a mercury lamp or a sodium lamp as the light source, the interference phenomenon of the film can be observed. If the inner surface of the glass is not very flat and the thickness of the air layer is uneven, irregular interference fringes of equal thickness will be observed, Usually, it is some irregular concentric rings. If a very flat glass sheet (such as a microscope slide) is used, parallel stripes will appear. When fingers press the glass sheet tightly, the thickness of the air film changes, and the stripes also change. Based on this principle, the flatness of the plane can be measured. The accuracy of the measurement is very high, Even protrusions or indentations as small as a fraction of the wavelength can be detected from the curvature of the stripes. If there is a small angle between two very flat glass plates, a wedge-shaped air film is formed. The interference fringes generated by monochromatic light with known wavelengths can be used to measure very small lengths
Thin film interference can also be used to manufacture anti reflective films. Coating a transparent film on the surface of the lens of a camera or projector can reduce light reflection and increase light transmission. This type of film is called an anti reflective film. There is usually a layer of blue purple reflection film on the camera lens, which is called an anti reflective film.