The principles of Huygens-Fresnel: interference, diffraction, polarization of light
The principles of Huygens-Fresnel became the basis of the wave-wave theory of light. At the beginning of the 19th century, Christian Huygens, making experiments on light waves, suggested that there are particles that are carriers of "light energy." This process was presented to him as a sequential transfer of energy from one corpuscle to the next through collisions. Scientists who supported this theory, argued that the light moves the ether, a medium with special physical properties that allow particles not to lose energy when moving. This ether permeates the entire surrounding space, and also passes through objects, allowing light waves to spread in all directions.
Fundamentals of Theory
What the principles of Huygens-Fresnel were based on can be formulated as follows: the propagation of light lies in the fact that light excitation,emanating from the light source, is transmitted to neighboring points in space, which generate secondary light waves and transmit them to neighboring points. The fields of propagation of secondary waves from neighboring points are superimposed on each other, amplifying or decaying. This theory is confirmed by the diffraction, interference, dispersion and reflection, which will be discussed in more detail below.
When two light waves overlap, they can either act as an amplifying factor or weaken each other's oscillations. The discovery of this phenomenon occurred seventeen years before the formulation of the principle of Huygens, in 1801 by Thomas Jung, an Englishman, a doctor by training. The scientist noted that if two very small holes were punctured next to each other on the cardboard and the screen was placed in the path of a narrow beam of light waves, such as a gap in the curtain, then there would be several light and dark rings on the wall behind the screen. In order for the experience to be successful, only one condition is necessary - light waves must be matched in their vibrations.
The light wave, passing through aerosols, liquids or solids, may deviate from the straight axis of motion. This phenomenon is called diffraction. It is used in optical devices to obtain a clear image of even the smallest objects, or objects at a considerable distance.
Simultaneously with Huygens, in 1818, Fresnel made a presentation of the diffraction report to the Paris Scientific Society. His experience and theoretical calculations were approved, and one of the commission members, the physicist Poisson, concluded on the basis of this theory that if an opaque round obstacle was placed in the path of diffraction-deflected rays, then a bright spot would be reflected on the screen, not the shadow of the object. Later this assumption was verified empirically by physicist D.F. Arago. The diffraction of light (the Huygens-Fresnel principle) was confirmed through what seemed to contradict the hypothesis. The wave theory of light has taken its place among other verified postulates of physics.
In addition to diffraction and interference, the principles of Huygens-Fresnel include the phenomenon of dispersion.In fact, it is the decomposition of a beam of light into individual waves after passing through an aerosol, liquid or solid. This phenomenon was discovered by Isaac Newton during his experiments with a prism. The splitting of light can be explained by the fact that the white beam consists of light waves of different lengths. Passing through an obstacle, the light is reflected at different angles, since the reflection coefficient is directly dependent on the wavelength. Because of this, waves of the same length form separate beams, which we perceive in a different color spectrum: from red to violet.
It is quite difficult to explain this physical principle. For more clarity, you can use the experience of the passage of light between two prisms. Its essence is that if solid transparent bodies are oriented in the same way, the light passes through them without losing its brightness, but if you put them perpendicular to each other, then the beam will not pass. This is explained by the fact that the vector of directivity of light waves. If it coincides with the plane on which the crystal is located, then there is no attenuation, and if it does not, then the light beam becomes less bright or does not pass through the object at all, in view of the fact that some of the waves are extinguished.
If a solid or liquid body appears on the path of a light wave, then it is fully or partially reflected by it. Thus, we can see the objects around us. When a light wave reaches the interface of media (for example, gas / liquid or gas / solid), it is fully or partially reflected back. The angle that forms between the beam of light and the perpendicular, pubescent on the surface (phase boundary) is called the angle of incidence, and the angle between the perpendicular and the reflected beam is the angle of reflection.
The laws of reflection:
- The incident and reflected rays and perpendicular exist in the same plane.
- The angle of incidence is equal to the angle of reflection.
- The course of the light rays is reversible.
Diffuse and mirror reflection
Depending on the type of surface from which the beam is reflected, it is possible to distinguish specular and diffuse reflection. Mirror is the reflection that is observed from a very smooth surface, when the irregularities do not exceed the wavelength. Then the reflected beam will be parallel to the incident. It is found in mirrors, glass, polished metal. If the surface irregularities are greater than the light wavelength, then the reflected rays are directed at different angles relative to the angle of incidence.It is because of this that we can see objects that are not sources of light. For the first time to come to this conclusion helped the principle of Huygens. The law of reflection of light received a mathematical and practical justification, based on already known concepts of interference and diffraction.
The principles of Huygens-Fresnel formed the basis for the design of optical devices, and also became the basis of the wave-wave theory of light. The Englishman D. Tabor, the Nobel Prize winner in physics, using this law, invented holography. Although its practical implementation became possible only with the introduction into mass use of narrowly focused intense light sources - lasers. In fact, a hologram is a picture of interference captured on a photographic plate, formed by light waves that reinforce and weaken each other, reflecting from the object at different angles.
The technique of capturing a three-dimensional image is used in the field of information storage, because more data is placed on a small surface of a hologram than on microphotographs.As an illustrative example, one can cite the location of an encyclopedic dictionary of one thousand three hundred pages on a photographic plate 3x3 cm.
Such devices as a holographic electron microscope are being developed, which allows creating three-dimensional images of the smallest structural units of living matter, as well as holographic cinema and television, the first versions of which are 3D-film shows.