— a chemical element, atomic number $9$, atomic mass $18.99840$. Light yellow gas with a pungent odor, density $1.693\ \mathrm{kg} / \mathrm{m}^{3}$, boiling point $−188.12\ ^{\circ} \! {\mathrm{C}}$, malting poimt $−219.6\ ^{\circ} \! {\mathrm{C}}$.

— the same as the Wave surface.

— areas into which wave surface when considering Fresnel diffraction. F. with. are selected so that the distance of each subsequent zone from the observation point was half wavelength greater than the distance of the previous zone from the same point. Breaking wave on the surface of the AF. based on theHuygens-Fresnel principle and can solve simple problems in the case of diffraction of spherical waves.

— a set of diffraction phenomena, considering them not neglect the curvature of the wave surface incident and diffracted (or just diffracted) waves. F. d. Observed in cases where the light source and the screen (or the screen only) are at finite distances from obstacles that causes diffraction. When F. d. The screen goes «diffraction image» noise. Investigated in $1815$ France. A. scientist Jean Fresnel $(1788 - 1827)$ (see. as Diffraction of light).

— a set of diffractive effects when considering which neglect the curvature of the wave surface incident and diffracted waves. F. d. Observed in cases where the light source and the screen is infinitely remote from obstacles, at which the diffraction. When F. d. The screen is obtained “diffraction picture” of the light. Research in $1814 - 1815$ it. I. optician Fraunhofer $(1787 - 1826)$ (see. as Diffraction of light).

— a radioactive chemical element, atomic number $87$ and mass number $223$. The metal density $2500\ \mathrm{kg} / \mathrm{m}^{3}$, melting point: $\sim 8\ ^{\circ} \! {\mathrm{C}}$.

— experiments, which could prove the existence of discrete energy levels of electrons in atoms. F.-H. e. have shown that the collision of electrons with atoms of the past are able to perceive only a certain amount of energy and therefore produce electromagnetic radiation of certain frequencies. Delivered in $1913$, it. physicists John. Frank $(1882 - 1964)$ and H. Hertz $(1887 - 1976)$.

— of optical anisotropy and the related originally double refraction in optically isotropic solid initially at their deformation.

— quantum field electromagnetic radiation. According to quantum submission, electromagnetic waves — a stream of photons, the elementary particles with zero rest mass and moving with the speed of light$(c)$ in vacuum with energy $\varepsilon=h \nu$ and momentum $p=\dfrac{h \nu}{c}$, where $\nu$ — frequency waves, $h$ — Planck’s constant.

— luminescence that is excited by visible light or ultraviolet radiation.