Of the broad spectrum of electromagnetic radiation, only a tiny fraction is detected, consciously felt and interpreted by human vision. This portion of the spectrum is usually called visible light.
The phenomenon of an object's color is the result of several factors: the illuminant, the object itself and the receiver. Ultimately it has to do with the light that reaches the eyes, intact as produced by the source or spectrally transformed by the observed object.
The eyes contain on their inner posterior surface a set of four types of sensors to receive and transmit to the brain the information obtained from the images of objects formed there. Rods have a high sensitivity across the visible spectrum with a maximum in their central area, giving an integrated information of the incoming light. The other three types of sensors, the cones, are sensitive to different areas of the spectrum, thus giving conjugated information to the brain, resulting in a sensation we call color.
Objects can interact with light differently, depending on their atomic or molecular makeup. Thus, they can absorb light in a certain spectral zone, diffuse it or let it pass through.
As an example, the blue color of the sky results from the preferential diffusion that air particles cause in the blue zone of the visible spectrum, observing themselves transversally in relation to the direction of incidence of the sun's rays. For the same reasons, the orange color of the sky in the direction close to that of the sun is due to the fact that light, when passing through the atmosphere, 'loses' part of the radiation in the blue zone.
The phenomena of absorption and diffusion work together. So, as an example, if an object, illuminated with white light (covering the entire visible spectrum), essentially absorbs in the blue zone, its 'color' will be yellowish, or if it absorbs in the green zone, its 'color' is magenta (mixed blue and red).
If a white object is illuminated with blue light, the object appears to be blue in color. If, however, the same object is illuminated with red light, it is perceived as red. On the other hand, an object that only diffuses 'red' light and absorbs all the rest of the visible spectrum, if illuminated with white light, will look red, but if illuminated with blue light, it will appear black, as it absorbs this radiation. .
Objects can also 'acquire' color through other optical phenomena, such as reflection ('color' of the water surface in a lake, which reflects the images of objects outside), refraction (rainbow formed by the passage of the light from the interior of water droplets in the atmosphere) or by diffraction of light (on the surface of an oil slick, on the surface of a CD or DVD).
Finally, it should be considered that, throughout the year, sunlight falls on different inclinations, varying depending on the latitude of the place. Thus, in winter in Portugal and for the same scenario and time of day, sunlight passes through a greater amount of air in the atmosphere, making its color more yellow than in summer.
In addition, the greater amount of water in the atmosphere favors the formation of fog and clouds, causing less intensity of light to fall on the earth's surface and greater diffusion of light across its spectrum.
Objects existing in nature, namely trees and other plants, change their structure or molecular composition. For all these factors, the transition from summer to autumn is the time of the year when the change from 'warmer' colors to 'colder' colors is most noticeable.
Author Francisco Gil
(Department of Physics, University of Coimbra)
Science in the Regional Press – Ciência Viva
Photo: Antonio Piedade
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