The phenomenon of color

All colors are the friends of their neighbors and the lovers of their opposites.

When looking out of my office window on a nice sunny summers’ day, it is not the color landscape that dominates my first impression, but the light and the brightness. However at a second glance it is the colors which add the depth, they set the atmosphere and bring the world to life- The green of the trees and the bright blue of the sky.

But how do things become colourful?

The physical explanation of colors is quite simple; a certain color is just light at that given wavelength or frequency. In our world we are however rarely confronted with light of just a given wavelength. Natural light from the sun or synthetic light from light bulbs send out essentially all wavelengths of the electromagnetic spectrum, ranging from radio- and microwaves we know from our homes to x-rays used at the hospital and potential harmful gamma rays. What we then define as color is basically light within the visible spectra, having wavelengths from 400 nm (violet) to around 700 nm (red). This covers just a small part of the electromagnetic spectrum but it’s what we, with human eyes, can detect.

At a second glance it is the colors which add the depth, they set the atmosphere and bring the world to life

What then gives an object its specific color?

Why are the leaves green and my Manchester United coffee mug red? The color of my mug is actually not with the mug, as in the absolute darkness it would have no color. In a simplified manner when the sunlight or light from my lamp hits my mug, three things can happen for each individual stream of photons. Light can be absorbed, converting the energy of the light to heat (that’s why black t-shirts get extra hot). Light can also be transmitted, which means that light is passes through the medium, like light through a window. Lastly light, or streams of photons, can be reflected meaning that the light is sent back out into the world (here the white T-shirt does the trick).

Going back to the example of my Manchester United mug, the crest is mostly red, with a bit of yellow, and the letters of Manchester United are colored in black. So physically what happens is that the sunlight of all wavelengths hit my mug and most of these are absorbed adding no color to the mug. However, where the crest is, light of approximately 650-700 nm (red) is reflected (turning the mug red). Also, a small part of 630 nm (yellow)is reflected, bringing the logo of my favorite team to life. The black letters are black due to all light being absorbed with no reflection.

So the color of an object is actually the color of the light that has been reflected after it hit the object. But what causes this reflection? On a molecular level, compounds called pigments are molecules which reflect light rather than absorb or transmit it. So my ceramic mug is covered in various compounds that individually reflect red and yellow wavelengths. Looking outside my window again, the leaves on the trees are green because they contain chemical compounds or pigments called chlorophylls in such a high manner that reflects only light in the wavelength of green. In autumn when the leaves turn red, orange, yellow, and green, the chlorophyll in the leaves is replaced by other pigments. This happens in response to the changes of sunlight (shorter or longer days), water (rain) and other external factors. In autumn as chlorophyll production is slowed down, pigments such as carotenoids (making carrots orange) or anthocyanins (making berries red) begin accumulated in the leaves reflecting light of more orange and red wavelengths.

Color is an interesting matter. Because one thing is that a stream of photons at a certain wavelength is reflected off my mug making it appear red. Another crucial factor is that I am capable e to interpret this reflected light as a color. This color detection tool I have is located in my eye as my retina has photoreceptors, rods and cones. Rods are crucial for detecting darkness and light. They are extremely sensitive and can detect in the range of single photons, however they play little role in the color vision as they mostly active under very low light conditions. In bright daylight, however our cone photoreceptors are activated and let us detect colors. The human eye consists of three types of cones sensitive to short, medium, and long wavelengths of the visible spectrum. These three receptors transmit signals back to the brain telling us to perceive my Manchester United mug as red, yellow, and black. Other animals have different amounts and types of photoreceptors, for example dogs have two types of cones which are sensitive to yellow and blue. That means dogs can’t distinguish between green, red, and yellow object based on their color. It is similar to how a person suffering from red-green color blindness and perceive colors between red and green as one hue.