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The Nature of Light
The first thing we will look at is the nature of light. Light is part of the electromagnetic spectrum. Visible light is composed of photons. These photons are tiny packets of energy that essentially have no mass. Light is therefore pure energy. The dual nature of light makes it behave like a wave as well as like a particle. Passing light through a prism will break it up into its constituent spectral colors, each with it's own wavelength. This is important to note, since our atmosphere also acts like a giant prism. We see light getting refracted, reflected, scattered and dispersed by our atmosphere, which results in many different kinds of observable phenomenon that we see, from blue skies to rainbows to red sunrises and sunsets.
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Refraction and Dispersion of Light
Light changes speed when it travels from one medium, such as air, to another medium, such as water. It also changes direction as it gets bent by the medium. This is known as refraction of light. When star light enters the Earth's atmosphere, the air in the atmosphere refracts the light. Refraction will cause a slight shift in the position of the star or any other celestial body, which is a source of light. Dispersion of light, also referred to as chromatic dispersion, occurs when different wavelengths (colors) of light are separated and bend by different amounts. The longer the wavelength of a particular color, the less it will bend.
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Atmospheric Prismatic Dispersion
In astronomy, atmospheric prismatic dispersion is the refraction and dispersion of light by the atmosphere causing an apparent change in celestial bodies that we see. There is a direct relationship between atmospheric dispersion of an object and the angle at which it is elevated above the horizon. This is very important in understanding what causes the appearance of red and blue stars on the horizon.
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Red and Blue Stars On the Horizon
Now that we understand atmospheric prismatic dispersion, we can explain the appearance of red stars on the horizon, as well as red and blue stars on the horizon. As mentioned earlier, atmospheric dispersion depends on the angle of elevation above the horizon. Stars near the horizon will be dispersed more so than stars higher up in the sky. This is because light from the star has to pass through a thicker layer of the atmosphere near the horizon than it does at higher elevations. Red light bends the least, while blue light bends the most. So near the horizon, stars will appear to flash red as that is the color being dispersed the most, while others will appear to flash red and blue as those are the colors being dispersed by the atmosphere. As the Earth is constantly spinning on its axis, the angle of the star will also change resulting in less atmospheric dispersion higher up.
So the next time you are out on a clear night observing the heavens and you spot red stars on the horizon or red and blue stars on the horizon, remember that it is the atmosphere that is responsible for what you see.
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Image Credits: Wikimedia Commons:Jack Delano/Marcelo Reis/Andrew Dunn/Andy