ScienceIQ.com

The Ants Go Marching One by One, Hurrah!

Have you ever wondered how ants know the way from one place to another? Even when you remove them all, they are right back to the trail they were on before as if there were an invisible road telling them where to go! How do they do that? Well, actually, there are invisible roads telling them where to go, and they are called pheromone trails. ...

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AntsMarching
Astronomy

Blast Wave Blows Through the Solar System

Although the Sun provides the means for life on Earth, it has a dark side - the Sun regularly sends massive solar explosions of radiative plasma with the intensity of a billion megaton bombs hurtling ... Continue reading

BlastWaveSolarSystem
Biology

Microorganisms: Are they really that bad?

We buy antibacterial hand soaps and cleaners to get rid of microorganisms that we don't want around us or our homes, but can some of them actually be helpful? You may think that they only cause harm, ... Continue reading

Microorganisms
Biology

Obesity: How much fat can your genes handle?

According to some experts, the popular formula for weight loss, 'eat less, and exercise more,' is not working for many Americans. Recent estimates say that about 34% of adults and 22% of preschool ... Continue reading

Obesity
Engineering

Seeing In The Dark

In the movies, there are all sorts of nasty things that can see perfectly well in the dark. More realistic movies also boast their share of 'beasts' that can see in the dark. Who could forget the ... Continue reading

SeeingInTheDark

Unit Of Luminous Intensity (candela)

CandelaOriginally, each country had its own, and rather poorly reproducible, unit of luminous intensity; it was necessary to wait until 1909 to see a beginning of unification on the international level, when the national laboratories of the United States of America, France, and Great Britain decided to adopt the international candle represented by carbon filament lamps. Germany, at the same time, stayed with the Hefner candle, defined by a flame standard, and equal to about nine-tenths of an international candle. But a standard based on incandescent lamps, and consequently dependent upon their stability, would never have been fully satisfactory and could therefore be only provisional; on the other hand, the properties of a blackbody provided a theoretically perfect solution and, as early as 1933, the principle was adopted that new photometric units would be based on the luminous emission of a blackbody at the freezing temperature of platinum (2045 K).

The units of luminous intensity based on flame or incandescent filament standards in use in various countries before 1948 were replaced initially by the 'new candle' based on the luminance of a Planckian radiator (a blackbody) at the temperature of freezing platinum. This modification had been prepared by the International Commission on Illumination (CIE) and by the CIPM before 1937, and was promulgated by the CIPM in 1946. It was then ratified in 1948 by the 9th CGPM which adopted a new international name for this unit, the candela (symbol cd); in 1967 the 13th CGPM gave an amended version of the 1946 definition.

In 1979, because of the experimental difficulties in realizing a Planck radiator at high temperatures and the new possibilities offered by radiometry, i.e., the measurement of optical radiation power, the 16th CGPM (1979) adopted a new definition of the candela: the candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.