Growing up, I began to wonder what determined the color of a substance. Judging from large-scale models, I thought that atoms were different colors and that they had this as a characteristic, like our hair color is a gene in our DNA. It was not until high school Physics that I learned that colors come from the way we perceive light waves, and how knowledge of colors can be used to identify temperatures.
Colors and the visible light spectrum
Light is the part we can see of the electromagnetic spectrum, a range of wavelengths and frequencies for the different forms of energy that travel in waves called electromagnetic radiation. The spectrum goes from zero to infinity, and is divided into regions like the visible light spectrum, with wavelengths between 400 and 700 nanometers (comparable to the size of a molecule to a needle’s point). Colors are the names given to the properties our eyes perceive from visible light. In the visible spectrum, they range from low-frequency and high-wavelength red to the high-frequency and small-wavelength violet. From visible light of a single wavelength, the seven colors of the rainbow are obtained. A mix of different wavelengths produces the other colors we are able to perceive.
Incandescence
One of the ways energy is transformed into light waves is through incandescence. This is when a heated object produces photons (basic units of light) during the transformation of heat to light energy. The amount of light this object produces at certain wavelength is determined by its temperature. Because of this, temperature also determines the color the solid emits. The relation between these two properties gives the color temperature of the object (measured in Kelvin). As seen in the color temperature scale below, objects with color temperatures close to 1,000 K (a temperature where light has a peak in the infrared region of the spectrum and begins to show more visible light) emit a red color. On higher color temperatures, the colors begin to lighten until around 5,400 K, which is the light yellow of direct sunlight. Over 10,000 K are the blue hues that characterize clear skies. Color temperatures are calculated considering a black-body radiator, which is an ideal object that radiates all the energy it absorbs. Black body radiators do not exist, but permit the finding of values to make color temperature scales for incandescent objects.
The colors of incandescent light sources
Knowledge about colors can be applied to estimating the temperature of light sources. A regular candle flame with a red orange flame is around 1,900 K. The common Tungsten light bulb which gives an orange glow has a temperature of 2,850 K. Colors are very important in determining the surface temperature of stars, as well as estimating their age based on the mass. With a surface temperature of 5,500 K, the Sun is an intermediate-mass yellow star around the beginning of the middle part in its lifecycle. As it continues to cool, it will eventually become a red giant, with a temperature of around 3,500 K for a bright red color, then past 2,500 K for darker red. After this, the Sun will lose its outer layers and become a planetary nebula, then a white dwarf. These two have temperatures of over 10,000 K and 100,000 K respectively, presenting blue colors. In the end of its cycle, the star cools slowly until it becomes a hard-to-detect black dwarf and fades. This is one of the ways that understanding colors helps in understanding the world around us.
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