why do we have a red sunset

The sun emits light waves with a range of frequencies. Some of these frequencies fall within the
and thus are detectable by the human eye. Since sunlight consists of light with the range of visible light frequencies, it appears. This white light is incident towards Earth and illuminates both our outdoor world and the atmosphere that surrounds our planet. As discussed, the interaction of visible light with matter will often result in the absorption of specific frequencies of light. The frequencies of visible light that are not absorbed are either transmitted (by transparent materials) or reflected (by opaque materials). As we sight at various objects in our surroundings, the color that we perceive is dependent upon the color(s) of light that are reflected or transmitted by those objects to our eyes. So if we consider a green leaf on a tree, the atoms of the chlorophyll molecules in the leaf are absorbing most of the frequencies of visible light (except for green) and reflecting the green light to our eyes. The leaf thus appears green. And as we view the black asphalt street, the atoms of the asphalt are absorbing all the frequencies of visible light and no light is reflected to our eyes. The asphalt street thus appears. In this manner, the interaction of sunlight with matter contributes to the color appearance of our surrounding world. In this part of Lesson 2, we will focus on the interaction of sunlight with atmospheric particles to produce blue skies and red sunsets. We will attempt to answer these two questions: Why are the skies blue? Why are the sunsets red? Why are the skies blue? The interaction of sunlight with matter can result in one of three wave behaviors:. The atmosphere is a gaseous sea that contains a variety of types of particles; the two most common types of matter present in the atmosphere are gaseous nitrogen and oxygen. These particles are most effective in scattering the higher frequency and shorter wavelength portions of the visible light spectrum.


This scattering process involves the absorption of a light wave by an atom followed by reemission of a light wave in a variety of directions. The amount of multidirectional scattering that occurs is dependent upon the frequency of the light. (In fact, it varies according to f. ) Atmospheric nitrogen and oxygen scatter violet light most easily, followed by blue light, green light, etc. So as white light (ROYGBIV) from the sun passes through our atmosphere, the high frequencies (BIV) become scattered by atmospheric particles while the lower frequencies (ROY) are most likely to pass through the atmosphere without a significant alteration in their direction. This scattering of the higher frequencies of light illuminates the skies with light on the BIV end of the visible spectrum. Compared to blue light, violet light is most easily scattered by atmospheric particles. However, our eyes are more sensitive to light with blue frequencies. Thus, we view the skies as being blue in color. Why are sunsets red? Meanwhile, the light that is not scattered is able to pass through our atmosphere and reach our eyes in a rather non-interrupted path. The lower frequencies of sunlight (ROY) tend to reach our eyes as we sight directly at the sun during midday. While sunlight consists of the entire range of frequencies of visible light, not all frequencies are equally intense. In fact, sunlight tends to be most rich with yellow light frequencies. For these reasons, the sun appears yellow during midday due to the direct passage of dominant amounts of yellow frequencies through our atmosphere and to our eyes. The appearance of the sun changes with the time of day. While it may be yellow during midday, it is often found to gradually turn color as it approaches sunset.


This can be explained by light scattering. As the sun approaches the horizon line, sunlight must traverse a greater distance through our atmosphere; this is demonstrated in the diagram below. As the path that sunlight takes through our atmosphere increases in length, ROYGBIV encounters more and more atmospheric particles. This results in the scattering of greater and greater amounts of yellow light. During sunset hours, the light passing through our atmosphere to our eyes tends to be most concentrated with red and orange frequencies of light. For this reason, the sunsets have a reddish-orange hue. The effect of a red sunset becomes more pronounced if the atmosphere contains more and more particles. The presence of sulfur aerosols (emitted as an industrial pollutant and by volcanic activity) in our atmosphere contributes to some magnificent sunsets (and some very serious environmental problems). The Wonders of Physics The light that we see looks white, but if you look through a prism you will see that light is actually divided up into many colors. These are the colors of the light вspectrumв and are the same ones that we see in a rainbow. We see all of the colors, with the exception of вindigoв with our eyes: ROY G BIV: red, orange, yellow, blue, indigo, violet. This is important, because everything that we look at seems to have color, including a sunset. Each color that we can see has a вwavelengthв. The longer wavelengths are found in colors such as red, green, yellow and orange and the shorter wavelengths are the blue colors. The longest wavelength color is red. When light flows through space from the sun it is going in a straight line and has all of the colors of the spectrum. Each color is traveling at its own wavelength. Some are slow and some are fast. By the time the light reaches the earthвs atmosphere it may encounter water molecules, dust and ice.


Since the visible light waves are incredibly small (smaller than one millionth of a meter) the light waves will interact with even the tiny gas molecules that are in the air. They begin to bounce off of the particles in the same way you might bounce around in a busy school hallway in between classes. This bouncing process is called вscatteringв and where they go depends on how large the particle was that they hit when compared to the wavelength size. Small particles (compared to the wavelength) will scatter the blue light a lot more strongly than red light. Since the earthвs atmosphere is mostly made up of oxygen and nitrogen, the blues will scatter sunlight in more directions than the red. This creates the вblue skyв that we see. The red light waves have the least amount of scattering due to the interaction with the gas molecules. So when you are outside at sunrise or sunset the sunlight is actually travelling a longer path, through the atmosphere and finally to your eyes where you can see it. The blue light has been almost completely removed, but that still leaves the red and yellow light. This is why when you see a sunset, you can see the beautiful colors of reds, oranges and yellows. If there are any clouds, they will reflect these colors, creating an even prettier picture. The colors are being reflected from the water and ice particles in the clouds and the molecules also absorb the wavelength colors as well. We then wait until full daylight, when we can see the same process, but now all of the blue colors are being scattered and with so much more blue, we donвt see the reds, oranges or yellows any more There actually are a lot more вcolorsв в but as humans, we canвt see them. There are specialized equipment that scientists use to вseeв these colors and this helps them in various scientific studies.

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