If you take a look at light through a prism, take note on all of the different colors you can see. The white light that is visible to our naked eye is actually composed of several different colors and each color can be looked at as light waves all with different wavelengths. As we examine the different wavelengths (or colors) we determine that red is the longest and purples are the shortest and colors such as orange, yellow, blue and green lie in between red and purple on the visible spectrum. So using these simple concepts,  a question that is commonly asked by curiosity is why do we see sunsets as red?

When light comes from the Sun to Earth, all of the light waves with the different wavelengths are traveling through empty space in the atmosphere. When the waves reach the Earth’s atmosphere, the light waves interact with with air particles such as dust, water, ice crystals and even in some cases gas as well. The light will bounce off of all of theses different variables that remain in the air and the light then becomes scattered in all different directions that it has bounced. When a light wave gets scattered, it merely depends on the size of the particle compared strictly to the wavelength of the specific light. In the visible range of wavelengths, red light happens to be scattered the least by these free molecules in the atmosphere. So when it comes time for our eyes to view sunrise and sunset, our naked eye views mostly reds because at this point the blue light has mostly been removed due to the long path the light must travel.

The appearance of the sun then changes throughout the time of day. As it appears to us as yellow during the midday, it gradually changes colors as it approaches sunset. Which ties into the previously discussed concept of scattering. As the sun then approaches the horizon line, light must then travel a greater distance through the atmosphere. As this pathway light must travel increases length, ROYGBIV comes across tons and tons more particles in the air. This then obviously results in the yellow light (from the sun) being scattered more than ever before in the day. Of course then during the sunset hours, the light traveling in our atmosphere to our eyes tends to be more focused on seeing reds (and orange) frequencies of light. The sunset also becomes more pronounced if the atmosphere at the times contains more particles (water, dust, gas, ice crystals) because then the light will scatter even more.

Concluding these concepts, at sunrise and sunset the sun is obviously very low in the sky. Specifically at the horizontal line. This means that the sunlight we see has travelled through a much thicker atmosphere than during midday. Blue light is scattered more strongly by the atmosphere, the color is scattered several times more often and reflected in different directions than in our own. This simply means that there is more yellow and red wavelengths for us to see. The red wavelengths are always travelling in the same direction, and the blue changeup directions. Depending on what direction the blue is in, you’ll see red sunset or blue sky.

Sunsets are a limited time everyday when the sun is slowly exiting the sky for the night. Sunsets fascinate a lot of people leaving them curious on several factors about them. One of those being, why the sunsets appears to the naked eye as red? This question attracted my attention and interest simply because when I was a little girl my father and I would sit on our deck swing every night before bed and watch it go down. When I was younger, I didn’t think anything of it. However, growing up and learning about light this unit, had me wonder why the sky becomes red when the sun goes down. It is simply because red light waves are scattered the least by atmospheric molecules. So at sunrise and sunset, when the sunlight travels a longer path through the atmosphere to reach our eyes, the blue light has been pretty much removed because of its short wavelength leaving red and yellow light remaining in the sky.

Photo by: Digital Photography School


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