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For the Same Reason It Shows Red, the Sky is Blue. And the Color of Energy?

Oddly enough the setting sun shows red for the same reason the sky is blue.  And to understand why that is the case, we need to step back and learn a few things – first about light, and then about the Earth’s atmosphere. And how both light and atmosphere interact with each other, and how the colors red and blue might better inform us about the use of energy.

Visible Light

Visible light is a very small part of the electromagnetic spectrum that ranges from the longer wavelengths of the infrared, microwave, and radio waves to the shorter ultraviolet, x-ray and gamma rays. These are all a form of energy that travels in waves.  What we call visible light is the incredibly small part (about one thousandth of one percent) of the full electromagnetic spectrum that our eyes can see.  And while light from the sun (or from other sources as from a light bulb) may look white, it is actually a combination of many colors that blend into white light.

At one end of the spectrum white light includes the reds and oranges. They gradually shade into yellow and green, and finally into blue, indigo and violet.  Each color has a different wave length and different frequency of that wave.  Within visible light, the reds have the longest wavelength but the lowest frequency and energy.  Violet light has the shortest wavelength but also the lowest frequency and energy level.

As long as nothing disturbs it, light travels in a straight line.  But as it moves through our atmosphere the light bumps into a gas molecule or a bit of dust. And here we next we learn something about our atmosphere.

The Atmosphere

What we call the atmosphere is actually a mixture of gas molecules and other materials that surround the earth. Compared to the size of the earth it is a paper thin composition that is mostly made of the gases nitrogen (78%), and oxygen (21%). Argon gas and water (in the form of vapor, droplets and ice crystals) are the next most common things. There are also small amounts of other gases, including the heat-trapping carbon dioxide emissions. And there are many small solid particles or aerosols like dust, soot and ashes, pollen, and salt from the oceans.

The composition of the atmosphere depends on location, the weather, and many other things. There may be more water in the air after a rainstorm, or near the ocean. Volcanoes can put large amounts of dust particles high into the atmosphere. Pollution can add different gases or dust and soot. The atmosphere is densest near the Earth itself. It gradually thins out as we move higher up. There is no sharp break between the atmosphere and space.

The Interaction of Light and Atmosphere

Light travels through space in a straight line – as long as nothing disturbs it. As it moves through the atmosphere, however, it bumps into many bits of dust and the many gas molecules. What then happens depends on the wave lengths of the light and the size of the things they hit.

Water droplets, dust and pollution particles are larger than the wavelength of visible light. When light hits these large particles, it gets bounced off into different directions. The different colors of light are all reflected by the particle in the same way. The reflected light, as in a fog, appears more white because it still contains all of the same colors. (more…)