Application of interference, thin film interference

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

Today we will talk about the use of interference in science and everyday life, reveal the physical meaning of this phenomenon and tell about the history of its discovery.

Definitions and distributions

Before talking about the significance of a phenomenon in nature and technology, first you need to give a definition. Today we are considering a phenomenon that schoolchildren study in physics lessons. Therefore, before describing the practical application of interference, let's turn to the textbook.

To begin with, it should be noted that this phenomenon applies to all types of waves: those that arise on the surface of water or during research. So, interference is an increase or reduction in the amplitude of two or more coherent waves, which occurs if they meet at one point in space. The maxima in this case are called antinodes, and the minima are called nodes. This definition includes some properties of oscillatory processes, which we will reveal a little later.

The picture that results from superimposing waves on top of each other (and there can be a lot of them) depends only on the phase difference in which the oscillations come to one point in space.

Light is also a wave

Scientists came to this conclusion already in the sixteenth century. The foundations of optics as a science were laid by the world famous English scientist Isaac Newton. It was he who first realized that light consists of certain elements, the amount of which determines its color. The scientist discovered the phenomenon of dispersion and refraction. And he was the first to observe the interference of light on lenses. Newton studied such properties of rays as the angle of refraction in different media, double refraction, and polarization. He is credited with the first application of wave interference for the benefit of mankind. And it was Newton who realized that if light were not vibrations, it would not exhibit all these characteristics.

Light properties

The wave properties of light include:

1. Wavelength. This is the distance between two adjacent highs of one swing. It is the wavelength that determines the color and energy of visible radiation.
2. Frequency. This is the number of complete waves that can occur in one second. The value is expressed in Hertz and is inversely proportional to the wavelength.
3. Amplitude. This is the "height" or "depth" of the oscillation. The value directly changes when two oscillations interfere. The amplitude shows how much the electromagnetic field was disturbed in order to generate this particular wave. It also sets the field strength.
4. Wave phase. This is the part of the oscillation that is reached at a given time. If two waves meet at the same point during interference, then their phase difference will be expressed in units of π.
5. Coherent electromagnetic radiation is called withthe same characteristics. The coherence of two waves implies the constancy of their phase difference. There are no natural sources of such radiation, they are created only artificially.

First application is scientific

Sir Isaac worked hard and hard on the properties of light. He observed exactly how a beam of rays behaves when it encounters a prism, a cylinder, a plate, and a lens from various refractive transparent media. Once, Newton placed a convex glass lens on a glass plate with a curved surface down and directed a stream of parallel rays onto the structure. As a result, radially bright and dark rings radiated from the center of the lens. The scientist immediately guessed that such a phenomenon can be observed only if there is some periodic property in the light that quenches the beam somewhere, and somewhere, on the contrary, enhances it. Since the distance between the rings depended on the curvature of the lens, Newton was able to approximately calculate the wavelength of the oscillation. Thus, the English scientist for the first time found a concrete application for the phenomenon of interference.

Slit interference

Further studies of the properties of light required setting up and conducting new experiments. First, scientists learned how to create coherent beams from fairly heterogeneous sources. To do this, the flow from a lamp, candle or sun was divided into two using optical devices. For example, when a beam strikes a glass plate at an angle of 45 degrees, then part of itis refracted and passes on, and part is reflected. If these streams are made parallel with the help of lenses and prisms, the phase difference in them will be constant. And so that in the experiments the light did not come out like a fan from a point source, the beam was made parallel using a close-focus lens.

When scientists learned all these manipulations with light, they began to study the phenomenon of interference on a variety of holes, including a narrow slit or a series of slits.

Interference and diffraction

The experience described above became possible due to another property of light - diffraction. Overcoming an obstacle small enough to be compared with the wavelength, the oscillation is able to change the direction of its propagation. Due to this, after a narrow slit, part of the beam changes the direction of propagation and interacts with beams that did not change the angle of inclination. Therefore, the applications of interference and diffraction cannot be separated from each other.

Models and reality

Up to this point, we have used the model of an ideal world in which all beams of light are parallel to each other and coherent. Also, in the simplest description of interference, it is implied that radiations with the same wavelengths are always encountered. But in reality, everything is not so: light is most often white, it consists of all the electromagnetic vibrations that the Sun provides. This means that interference occurs according to more complex laws.

Thin films

The most obvious example of this kindinteraction of light is the incidence of a beam of light on a thin film. When there is a drop of gasoline in a city puddle, the surface shimmers with all the colors of the rainbow. And this is precisely the result of interference.

Light falls on the surface of the film, is refracted, falls on the border of gasoline and water, is reflected, and refracted again. As a result, the wave meets itself at the exit. Thus, all waves are suppressed, except for those for which one condition is satisfied: the film thickness is a multiple of a half-integer wavelength. Then at the output the oscillation will meet itself with two maxima. If the thickness of the coating is equal to the whole wavelength, then the output will superimpose the maximum on the minimum, and the radiation will extinguish itself.

From this it follows that the thicker the film, the greater must be the wavelength that will come out of it without loss. In fact, a thin film helps to highlight individual colors from the entire spectrum and can be used in technology.

Photo shoots and gadgets

Oddly enough, some applications of interference are familiar to all fashionistas around the world.

The main job of a beautiful female model is to look good in front of the cameras. A whole team prepares women for a photo shoot: a stylist, makeup artist, fashion and interior designer, magazine editor. Annoying paparazzi can lie in wait for a model on the street, at home, in funny clothes and a ridiculous pose, and then put the pictures on public display. But good equipment is essential for all photographers. Some devices can cost several thousand dollars. AmongThe main characteristics of such equipment will necessarily be the enlightenment of optics. And the pictures from such a device will be of very high quality. Accordingly, a star shot without preparation will also not look so unattractive.

Glasses, microscopes, stars

The basis of this phenomenon is interference in thin films. This is an interesting and common phenomenon. And finds light interference applications in a technique that some people hold in their hands every day.

The human eye perceives green color best. Therefore, photographs of beautiful girls should not contain errors in this particular region of the spectrum. If a film with a specific thickness is applied to the surface of the camera, then such equipment will not have green reflections. If the attentive reader has ever noticed such details, then he should have been struck by the presence of only red and purple reflections. The same film is applied to glasses glasses.

But if we are talking not about the human eye, but about a passionless device? For example, a microscope must register the infrared spectrum, and a telescope must study the ultraviolet components of stars. Then an anti-reflection film of a different thickness is applied.