Руслан Губайдуллин

Optics is a branch of physics that studies the behavior and properties of light, including its interaction with matter and the creation of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared radiation. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves have similar properties.

Most optical phenomena can be explained using classical electrodynamics. However, the full electromagnetic description of light is often difficult to apply in practice. Practical optics is usually based on simplified models. The most common of these, geometric optics, views light as a set of rays that travel in straight lines and bend as they pass through or reflect from surfaces. Wave optics is a more complete model of light that includes wave effects such as diffraction and interference that are not taken into account in geometric optics. Historically, the ray model of light was developed first, and then the wave model of light. Progress in the theory of electromagnetism in the 19th century led to the understanding of light waves as the visible part of the electromagnetic spectrum.

Some phenomena depend on the fact that light exhibits wave and particle properties. The explanation for this behavior lies in quantum mechanics. When considering corpuscular properties, light is thought of as a set of particles called photons. Quantum optics uses quantum mechanics to describe optical systems.

Optical science is relevant and studied in many related disciplines, including astronomy, various fields of engineering, photography and medicine (especially ophthalmology and optometry). Practical applications of optics can be found in a variety of technologies and everyday things, including mirrors, lenses, telescopes, microscopes, lasers, and fiber optics.

The term pathogen came into use in the 1880s. Typically, the term is used to describe an infectious microorganism or agent, such as a virus, bacterium, protozoan, prion, viroid, or fungus.Small animals, such as certain worms or insects, can also cause or transmit disease. However, these animals are usually, in common parlance, referred to as parasites rather than pathogens. The scientific study of microscopic organisms, including microscopic pathogenic organisms, is called microbiology, while parasitology refers to the scientific study of parasites and the organisms that host them.

There are several pathways through which pathogens can invade a host. The principal pathways have different episodic time frames, but soil has the longest or most persistent potential for harboring a pathogen.

Diseases in humans that are caused by infectious agents are known as pathogenic diseases. Not all diseases are caused by pathogens, other causes are, for example, toxins, genetic disorders and the host's own immune system.