A post submitted by CGI member ScienceTruth.
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The Winnemucca Institute for Advanced Studies presents:
Why Does the Sun Appear Yellow
The short answer : Because the Sun radiates primarily in the yellow region of the visible light portion of the ElectroMagnetic Spectrum, and because our eyes are biased towards producing a larger response to green-yellow light frequencies due to how our three different retinal color sensing cones are orchestrated.
To properly begin the long explanation it is first necessary to specify how Physics describes color, how the human eye interprets color energy, and how the brain interprets the eye's transmitted information.
Sir Isaac Newton discovered in 1667 that light could be split into many colors by a prism, he used his experiments to further analyze light over the next decade. The colors produced by light passing through a prism are arranged in a precise array, or spectrum, from red, through orange, yellow, green, blue, indigo and into violet. A memory trick to recall is the name 'Roy G. Biv' where each letter represents a color. The order of the colors is important, and each color has a unique signature identifying its location in the color spectrum. The identifier of color is its wavelength, and one-wavelength is the distance from one wave-top to the next wave's top.
Almost 200 years after Newton's discovery, Michael Faraday in 1845 discovered that light passing thru a transparent dielectric medium could be re-polarized by a magnetic field ! This was the first experimental evidence that light and electromagnetism are related. James Clerk Maxwell in the 1860's and 70's correctly theorized that light was a form of ElectroMagnetic (EM) radiation, and that this EM spectrum of radiations contained: long wavelength radio waves, then the shorter and shorter; infrared, visible light, ultraviolet light, and up into the shortest of wavelengths the X-rays and gamma rays.
The radiated energy emitted by our Sun spans almost the entire EM spectrum. The visible light region of the EM spectrum however, only occupies a very small portion of the total EM spectrum, and the Sun in the visible light region has its maximum intensity in the yellow portion of that spectrum. Our eyes are also biased toward the color yellow in their structural design.
Light : Waves (wavelength and frequency) and Photons (particles)
Physics considers light to be an ElectroMagnetic wave, and what determines a color is its wavelength. Our eyes only sense the range of wavelengths that we call the 'visible light' portion of the EM spectrum. And although radiations of longer and shorter wavelengths exist, the human eye is not capable of responding to them. The visible light spectrum goes from Red, at around 700 nanometers (nm) in wavelength, to Violet at around 400 nm. The shorter the wavelength, the higher the frequency, and the larger the energy content.
The nanometer is a unit of distance in the metric scale and is abbreviated as nm. One nanometer (nm) equals one thousandth of one millionth of a meter, or one billionth of a meter (m), or 1 nm = 10^-9 m. One nanometer is a distance too small to be resolved in an optical microscope, but one micron (um) or one-thousand nanometers can be resolved (1 micron = 1000 nm or 10^-6 m). The wavelengths of visible light are smaller than common objects such as the thickness of a sheet of paper or the diameter of a human hair. Both of these are about one-hundred microns thick which translates to distances greater than one-thousand to two-thousand wavelengths of visible light depending on the color. Or, one to two thousand times bigger than one wavelength of visible light.
We can describe light as electromagnetic waves with each color identified by its wavelength. We can also describe light as a stream of minute packets of energy, 'packets' which Physics calls 'photons', these photons create a pulsating electromagnetic disturbance upon impingement with a surface. A single photon of one color differs from a photon of another color, only by its inherent energy level. In a photon description of light, the most convenient unit of energy to use for these differing photon energy levels is the electron volt, or "eV". One electron volt is the energy gained by an electron that moves across a voltage differential of one volt. For example, 1.5 eV is the energy gained by an electron moving from a negative plate to a positive plate, which are connected to their respective terminals on a common 1.5 volt D-cell battery.
The Color of Objects
In the EM spectrum the visible light region extends from 700 nm (Red) to 400 nm (Violet) with corresponding photon eV energies of 1.8 eV for Red, to 3.1 eV for Violet. As the frequency becomes shorter, the light has more energy, or more eV. Thusly, Red light has only 1.8 eV, while Violet has 3.1 eV. The 'cones' in the retina in our eye sense the eV level of the light energy, and that energy level is translated by the pigment coating in the cones, then the optic nerve sends that information to the brain to become our perception of color. So, our eyes are really measuring the eV energy level content of the EM spectrum photons, an energy level content which directly corresponds to its frequency/wavelength, and this is how we sense colors. Our color cones are not able to sense lower than 1.8 eV nor higher than 3.1 eV due to the limits of their electrical translator pigments, and this is what puts the limits on our range of perceptible colors. Any eV's immediately beyond our retinal cone's sensitivity range are the colors we cannot see, i.e. the lower Infrared (IR) and the higher Ultraviolet (UV). If our eyes had a wider range of sensitivity, then IR and UV could be seen. If wider than IR and UV, then we could see even further down and into the radiowave range, or further up into the x-ray range, and these would likewise conjure up as 'colors' as well.
Our perception of color arises from the energy level composition of the light which enters the eye. The retina on the inner surface of the back of the eye contains two types of photosensitive cells ; rods and cones. These cells contain pigments which absorb visible light. It is the cones that allow us to distinguish between different colors by differentiating the differing energy levels of the arriving photons. The rods however are most effective in dim light and mainly sense differences in 'flux' ; the volume of light, or aggregate light intensity. The rods do not sense a specific photon's discrete energy level or color. The intensity or brightness of light is determined by its flux, or number of photons per unit area, per unit of time; e.g. the number of photons per square centimeter, per second, and not the discrete energy level of any one, or many, of the photons. This is why in dim light we perceive colored objects as shades of grey, and not as shades of color.
In the retina, color is perceived by three different sets of cones, each having a main responsive sensitivity to a different photon energy level. The energy level sensitivities of these three different cones overlap slightly, cone R (pigment R) has a maximum sensitivity in the red-orange range, cone G (pigment G) in the green-yellow range, and cone B (pigment B) in the blue range. Due to the overlap between the R and G cones, and between the G and B cones, our eye is biased towards the green-yellow region, and this is a secondary reason for why we see the Sun as mainly being yellow. The primary reason is because the Sun shines predominantly in the yellow portion of the visible light region of the EM spectrum. There are other stars in our sky which shine mostly in the red, or mostly in the blue portion of the visible light region of the EM spectrum, and they do appear as mainly red or blue to our eyes.
White light is composed of all wavelengths of visible light, and when an object is illuminated by white light it can be thought of as absorbing all colors except the color of its appearance. For example, if white light shines on a pure blue object, it can be thought of as if the object absorbs all of the impinging photon energies except that of the blue color energy photons, and those it reflects. Or, one could postulate that the object absorbs all of the arriving photon energies and then re-radiates photons in only the blue energy range.
Each of the 'Roy G Biv' colors can be represented by a single wavelength of light, but 'complex colors' like purple or brown for example, cannot be represented by a single wavelength of light. Purple is composed of a mixture of light from the red and violet regions of the spectrum, and brown requires a mixture of wavelengths from at least three regions of the color sequence. These two 'complex colors', purple and brown, result from proportionate contributions from the three different energy level sensing cones in the retina. Other colors that are not 'single wavelength' colors, likewise result from various combinations of eV stimulations in the multiple retinal cones.
Information provided herein by the Winnemucca Institute for Advanced Studies is for educational purposes. Our ‘Man on the Street Series’ of informative Science Papers is designed to provide a semi-technical answer to everyday experiences.
