Today we talk about interference in thin films. The focus of our attention is the discovery, investigation and application of this remarkable physical phenomenon.
Definition
Before describing any law, you first need to understand what constituents enter into it. If this is not done, then the reader may skip important details, and the perception of a scientific fact will be distorted. A student who has missed one lesson in physics due to illness or laziness should definitely understand this topic on his own. Because each subsequent concept is based on the previous one. If one value is missed, the rest of the physics will be incomprehensible. Before proceeding to the conclusion of interference in thin films, one must first define the phenomenon.
This phenomenon can apply to any oscillatory processes. Waves of wind, sea and sound can interfere. The interaction occurs even in such complex quasiparticles as the collective vibrational lattice of crystals.
Interference is a phenomenon that occurs when several waves meet in one place. It consists in the fact that, when added, the amplitude of the resulting oscillation changes. This means that the waves can amplify, cancel each other or go further without changes.
Shine
The phenomenon of interference in thin films is the interaction of light waves. So before proceeding with the description of the phenomenon, it is necessary to explain the nature of these oscillations.
Light is a quantum of the electromagnetic field. A photon has the properties of both a wave and a particle. As long as a quantum moves through space, it is indestructible and eternal. Proof of this is the light of distant galaxies. Some of them may have already changed shape or even ceased to exist. But their radiation flew through space for billions of years, until it reached the gaze of people.
The main source of light is electronic transitions in an atom. A powerful thermonuclear reaction occurs inside the stars, as a result of which all types of electromagnetic radiation are released. Visible light is only a small portion of the entire scale that is accessible to human vision.
Wave properties
In order to describe briefly the interference in thin films, it is necessary to talk about the wave properties of light. To understand the shape of an ideal oscillation without attenuation, you just need to look at the graph of the sine or cosine in the usual Cartesian coordinates. The main properties of the photon are as follows:
- Wavelength. It is designated by the Greek letter λ. The wavelength is the distance between two identical phases. Most clearly, this value is demonstrated as the gap between two adjacent highs or lows.
- Frequency. It is designated differently depending on the type: the linear frequency is ν, the cyclic frequency is ω, and if this value is expressed as a function, then it is written in the Latin letter f , and certainly italic. The frequency and wavelength are related by the relation λ * ν = c, where c is the speed of light in vacuum. Thus, knowing one value, the other is very simple.
- Amplitude. For interference, this property of the wave is the most important. This is the height of the highs and lows of the swing. It is the amplitude that changes when two waves meet.
- Phase. For a single quantum, this factor does not matter. In the interaction, phase difference is important. The state (maximum, minimum, or desire for them), in which two waves came to one place, affects the final intensity during interference.
- Polarization. In general, this property describes the form of oscillation. The polarization of light is linear, circular and elliptical.
Refraction, reflection
The phenomenon of light interference in thin films is directly related to several other phenomena of linear optics.
Encountering an obstacle, light can act in different ways:
- to reflect;
- to break
- dissipate
- absorbed.
In the latter case, the photon gives its energy to matter, and there are some changes. Most often it’s just heating. No wonder the thing left in the sun becomes very hot. Many different quanta transmit their energy to a ball forgotten by children.
Scattering also implies that light interacts with matter: it is absorbed and re-emitted back. Often, emerging quanta have a different wavelength or polarization.
Refraction and reflection do not change the properties of the beam, the difference is only in the direction of light propagation.
All these processes are involved, for example, in imaging the surface of the lake.
Light behavior in thin coatings
The simplest example of a film coating is soap foam. Soap increases the surface tension of water. As a result, it forms very large areas with a small thickness. Soap bubbles shimmer with all the colors of the rainbow. And now we will explain why.
Light falls on the film. At the upper boundary of the coating, part of it is reflected, part is refracted. We are interested in the second beam, which turned out to be inside the substance. It reaches the bottom, and then also part is refracted, and part is reflected back into the film. The light that goes on next Wednesday is lost to the observer. But the one that returns back to the film is of interest to us, because at the border it refracts again and leaves on the first Wednesday from which it initially entered. It turns out that the incoming and outgoing beams are parallel to each other. This is one and the same light, only its phase at the exit has changed. The difference will determine what the observer sees: a light band or a dark one. The described process is the essence of interference in thin films. Newton's rings, which are observed in a parallel beam of light between a convex lens and a flat glass plate, are in fact of the same nature. They are very easy to observe: even students in physics classes can make this experience.
The distance between the light stripes
We hope that the reader has fully understood the mechanism of interaction of light and thin coatings. Now we give some formulas.
At the exit from the film, a picture of light and dark regions is observed. The areas where the final picture has the same illumination are called equal slope bands. The interference in thin films gives us the following formula for their calculation:
2m * λ = (2nh * cosβ ± λ) / 2.
Here: λ is the wavelength of the incident radiation, m is the order of interference, β is the angle between the beam refracted for the first time and the normal to the surface, n is the refractive index of the film, and h is its thickness.
It should be noted that this condition will show the geometric location of the points of the brightest areas of the interference pattern.
Thus, only those beams are located that fall on the surface of the film at the same angle. That is why they are called equal slope bands.
Cameras and Glasses
A schoolboy who finds physics a boring subject is probably asking himself the question: "Why is all this necessary?" Nevertheless, the interaction of light and thin coatings is used in everyday life quite widely.
On the lenses of any photo and television equipment there is a deposition: the thinnest transparent film. Its thickness is selected so that the camera does not give green glare (light of this wavelength damps itself, passing through a layer on the glass surface). This solution makes the image contrast and bright. After all, a person sees the green spectrum best and perceives the shortcomings of this color most clearly.
Translucent spraying is also applied to the lenses of microscopes and telescopes. And not necessarily the film thickness corresponds to green. If a scientist investigates processes with infrared or ultraviolet radiation, the equipment helps him in this particular range.
Lasers
Also, interference is used in lasers, but this fact is known to few.
Today, not a single type of human activity is complete without lasers. The device consists of three parts - pump, working fluid and reflector. The mirror is located at the ends of the main radiating material. Its purpose is to collect the generated photons of a specific wavelength in one direction. This element of the device is often a series of thin films, the interference on which allows only the desired radiation to pass on.