Light in Air
Light from the sun travels through the universe, i.e. space, this is a vacuum—that means there is no physical matter present through which light travels, or in other words, there is no medium through which light travels. However, as light is a type of electromagnetic wave form, it travels at an optimum speed when no obstacles are present. But once the light waves reach the Earth, they have to go through the Earth’s atmosphere, which does consist of physical matter. The atmosphere, consisting of layers of gases, the mixture of which we call ‘air,’ obstructs the path of light.
As we now know, light, when obstructed, behaves in strange manners. This is because it is a combination of rays with different frequencies, or as we put it, rays of different colors. So when the rays from the sun reach the Earth’s atmosphere, a phenomenon called Rayleigh Scattering occurs.
When light waves collide with a relatively big obstruction, one with a larger size than the wavelength of visible light, it bounces off, just like a ball bounces off a wall, and the wave is reflected in all directions, haphazardly. (diagram/animation) The light still appears to be ‘white,’ that is, a homogenous mixture of all the colors, because the light is all reflected in the same exact way. The light merely changes directions and gets diverted, because it is not passing through the material, unlike what happens when it enters a transparent object. But what happens if light waves collide with molecules that are much smaller than their wavelength? This is exactly what takes place when light waves enter the Earth’s atmosphere—the gas molecules are much smaller than the light waves.
The gas molecules, much smaller than light waves, absorb the light waves; a phenomenon called Rayleigh Scattering. But not all of the light gets absorbed, because, as we know, it has different frequencies and different wavelengths, and as a result, when the gas particles later radiate the absorbed wave energy, only certain frequencies get radiated. When we see the radiated frequencies only, we see that particular color, or the mixture of colors of the radiated frequencies. (link to color wheel) That is how we perceive color—not that we think about it while picking up a crayon to color something or pick the reddest apple to eat!
All of the frequencies can be absorbed, in fact, when they are all absorbed and radiated, we see ‘white.’ But most often, it is the higher frequency colors that are absorbed; colors like blue and violet. Lower frequency colors like red are not absorbed as often. Sometimes none of the light rays are absorbed, and that is what we call ‘black.’ So, in scientific terms, black is not a color at all, but an absence of color!
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