To find the total power radiated from an object, multiply by its surface area, A: In the still more general (and realistic) case, the emissivity depends on the wavelength. is the emissivity of the grey body if it is a perfect blackbody. The SI unit for absolute temperature T is the kelvin. The radiant emittance has dimensions of energy flux (energy per unit time per unit area), and the SI units of measure are joules per second per square metre, or equivalently, watts per square metre. The radiance from a specified angle of view (watts per square metre per steradian) is given byĪ body that does not absorb all incident radiation (sometimes known as a grey body) emits less total energy than a black body and is characterized by an emissivity, : Where k is the Boltzmann constant, h is Planck's constant, and c is the speed of light in a vacuum. The constant of proportionality σ, called the Stefan–Boltzmann constant, is derived from other known physical constants. Specifically, the Stefan–Boltzmann law states that the total energy radiated per unit surface area of a black body across all wavelengths per unit time (also known as the black-body radiant emittance) is directly proportional to the fourth power of the black body's thermodynamic temperature T: The Stefan–Boltzmann law describes the power radiated from a black body in terms of its temperature. The law follows from Fermat's principle of least time, which in turn follows from the propagation of light as waves.
Snell's law states that, for a given pair of media, the ratio of the sines of the angle of incidence θ1 and angle of refraction θ2 is equal to the ratio of phase velocities (v1 / v2) in the two media, or equivalently, to the refractive indices (n2 / n1) of the two media. The law is also satisfied in meta-materials, which allow light to be bent "backward" at a negative angle of refraction with a negative refractive index. In optics, the law is used in ray tracing to compute the angles of incidence or refraction, and in experimental optics to find the refractive index of a material. This law was named after the Dutch astronomer and mathematician Willebrord Snellius (also called Snell). Snell's law (also known as Snell–Descartes law and ibn-Sahl law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.
0 kommentar(er)
