Vidljivi spektar

Bela svetlost se disperguje prizmom u boje vidljivog spektra.

Vidljivi spektar je porcija elektromagnetnog spektra koja je vidljiva ljudskim okom. Elektromagnetan radijacija u ovom opsegu talasnih dužina se naziva vidljiva svetlost ili jednostavno svetlost. Tipično ljudsko oko reaguje na talasne dužine od oko 380 do 740 nanometara. Po frekvenciji, to odgovara opsegu u blizini 430–770 THz.

The spectrum does not contain all the colors that the human eyes and brain can distinguish. Unsaturated colors such as pink, or purple variations like magenta, for example, are absent because they can only be made from a mix of multiple wavelengths. Colors containing only one wavelength are also called pure colors or spectral colors.

Visible wavelengths pass largely unattenuated through the Earth's atmosphere via the "optical window" region of the electromagnetic spectrum. An example of this phenomenon is when clean air scatters blue light more than red light, and so the midday sky appears blue. The optical window is also referred to as the "visible window" because it overlaps the human visible response spectrum. The near infrared (NIR) window lies just out of the human vision, as well as the medium wavelength infrared (MWIR) window, and the long wavelength or far infrared (LWIR or FIR) window, although other animals may experience them.

Istorija

Newton's color circle, from Opticks of 1704, showing the colors he associated with musical notes. The spectral colors from red to violet are divided by the notes of the musical scale, starting at D. The circle completes a full octave, from D to D. Newton's circle places red, at one end of the spectrum, next to violet, at the other. This reflects the fact that non-spectral purple colors are observed when red and violet light are mixed.

In the 13th century, Roger Bacon theorized that rainbows were produced by a similar process to the passage of light through glass or crystal.

In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light, and described the phenomenon in his book Opticks. He was the first to use the word spectrum (Latin for "appearance" or "apparition") in this sense in print in 1671 in describing his experiments in optics. Newton observed that, when a narrow beam of sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into and through the glass, emerging as different-colored bands. Newton hypothesized light to be made up of "corpuscles" (particles) of different colors, with the different colors of light moving at different speeds in transparent matter, red light moving more quickly than violet in glass. The result is that red light is bent (refracted) less sharply than violet as it passes through the prism, creating a spectrum of colors.

Newton's observation of prismatic colors (David Brewster 1855)

Newton originally divided the spectrum into six named colors: red, orange, yellow, green, blue, and violet. He later added indigo as the seventh color since he believed that seven was a perfect number as derived from the ancient Greek sophists, of there being a connection between the colors, the musical notes, the known objects in the solar system, and the days of the week. The human eye is relatively insensitive to indigo's frequencies, and some people who have otherwise-good vision cannot distinguish indigo from blue and violet. For this reason, some later commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right but merely as a shade of blue or violet. Evidence indicates that what Newton meant by "indigo" and "blue" does not correspond to the modern meanings of those color words. Comparing Newton's observation of prismatic colors to a color image of the visible light spectrum shows that "indigo" corresponds to what is today called blue, whereas "blue" corresponds to cyan.

In the 18th century, Johann Wolfgang von Goethe wrote about optical spectra in his Theory of Colours. Goethe used the word spectrum (Spektrum) to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colors. Goethe argued that the continuous spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow and blue-cyan edges with white between them. The spectrum appears only when these edges are close enough to overlap.

In the early 19th century, the concept of the visible spectrum became more definite, as light outside the visible range was discovered and characterized by William Herschel (infrared) and Johann Wilhelm Ritter (ultraviolet), Thomas Young, Thomas Johann Seebeck, and others. Young was the first to measure the wavelengths of different colors of light, in 1802.

The connection between the visible spectrum and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century. Their theory of color vision correctly proposed that the eye uses three distinct receptors to perceive color.

Percepcija boja kod raznih vrsta

Takođe pogledajte: Color vision § Physiology of color perception

Many species can see light within frequencies outside the human "visible spectrum". Bees and many other insects can detect ultraviolet light, which helps them find nectar in flowers. Plant species that depend on insect pollination may owe reproductive success to their appearance in ultraviolet light rather than how colorful they appear to humans. Birds, too, can see into the ultraviolet (300–400 nm), and some have sex-dependent markings on their plumage that are visible only in the ultraviolet range. Many animals that can see into the ultraviolet range cannot see red light or any other reddish wavelengths. Bees' visible spectrum ends at about 590 nm, just before the orange wavelengths start. Birds can see some red wavelengths, although not as far into the light spectrum as humans. The popular belief that the common goldfish is the only animal that can see both infrared and ultraviolet light is incorrect, because goldfish cannot see infrared light. Similarly, dogs are often thought to be color blind but they have been shown to be sensitive to colors, though not as many as humans. Some snakes can "see" radiant heat at wavelengths between 5 and 30 μm to a degree of accuracy such that a blind rattlesnake can target vulnerable body parts of the prey at which it strikes, and other snakes with the organ may detect warm bodies from a meter away. It may also be used in thermoregulation and predator detection. (See Infrared sensing in snakes)

Spektar boja

Glavni članak: Disperzija (optika)

sRGB rendering of the spectrum of visible light
Boja	Talasna dužina	Frekvencija	Energija fotona
Ljubičata	380–450 nm	680–790 THz	2.95–3.10 eV
Plava	450–485 nm	620–680 THz	2.64–2.75 eV
Cijan	485–500 nm	600–620 THz	2.48–2.52 eV
Zelena	500–565 nm	530–600 THz	2.25–2.34 eV
Žuta	565–590 nm	510–530 THz	2.10–2.17 eV
Narandžasta	590–625 nm	480–510 THz	2.00–2.10 eV
Crvena	625–740 nm	405–480 THz	1.65–2.00 eV

Colors that can be produced by visible light of a narrow band of wavelengths (monochromatic light) are called pure spectral colors. The various color ranges indicated in the illustration are an approximation: The spectrum is continuous, with no clear boundaries between one color and the next.

Ako se uzak snop bele svetlosti propusti kroz pukotinu i da zatim prođe kroz optičko sočivo tako da zraci u paralelnom snopu padaju na optičku prizmu dolazi do njegovog razlaganja. Pri tom prizma mora biti nameštena na minimum devijacije. Nakon loma u prizmi taj će se uski snop svetlosti raširiti u široku prugu raznobojne svetlosti koja se zove spektar boja. Spektar bele svetlosti sastoji se od 6 boja i to: crvene, narančaste, žute, zelene, plave i ljubičaste koje neprekidno prelaze jedna u drugu. Ovo rastavljanje bele svetlosti u 6 spektralnih boja zove se disperzija svetlosti. Disperziju svetlosti je prvi istražio I. Njutn i time objasnio hipotezu da je bela svetlost sastavljena iz različitih, takozvanih spektralnih boja.

Kako se prizmom svetlost dvaput lomi, to do disperzije dolazi zato što svaka spektralna boja ima različiti indeks loma. Kod toga se najmanje lomi crvena, a najviše ljubičasta svetlost. Dakle, indeks loma ljubičaste svetlosti veći je od indeksa loma crvene svetlosti. Odatle izlazi da se crvena i ljubičasta svetlost šire u staklu različitim brzinama. Znači da brzina svetlosti u prozirnim sredstvima zavisi od boje svetlosti. Brzina crvene svetlosti je najveća. Sve manju brzinu ima po redu, narančasta, žuta, zelena, plava i modra svetlost, najmanju ljubičasta.

Merenja su pokazala da u vakuumu brzina svetlosti ne zavisi od njene boje. Stoga u vakuumu nema disperzije svetlosti.

Da je spektralna svetlost homogena i jednobojna (monohromatska), to jest da se ne može rastaviti, može se potvrditi eksperimentom. U zastoru na koji pada spektar boja napravi se uska pukotina tako da kroz nju prolazi snop jednobojne svetlosti i da pada na drugu prizmu. Druga prizma mora biti tako postavljena da joj lomni brid bude paralelan s lomnim bridom prve prizme. Zbog loma na prizmi svetlost će biti otklonjena, ali neće nastati disperzija.

Vidi još

• Visokoenergetska vidljiva svetlost
• Elektromagnetna apsorpcija vodom