Ask A Scientist

When the sky absorbs blue, what happens during a sunset or sunrise?

Asked by: Katelyn Rundell
School: Seton Catholic at All Saints School
Grade: 5
Teacher: Matthew Martinkovic
Hobbies/Interests: Hobbies: Singing, dancing, soccer, field hockey and softball
Career Interest: To sing on Broadway

Answer from Steve Rebello

Math/Science Academic Counselor

Before Binghamton University
Taught grade school physical science and biology in the Johnson City School district and high school biology and physics in the Upward Bound program at Binghamton University. Graduate School Research Area: Mycology, Phylgenetics
Wife, Emily - 5th grade teacher in Union-Endicott School District and son, Brayden 
Photography, camping, skiing, snowboarding and teaching  

To comprehend where all of the beautiful and brilliant colors in a sunset come from, it is first necessary to understand the properties of light. Light is one of the many forms of electromagnetic energy. X-Rays, Gamma rays, radio waves, and microwaves are also forms of electromagnetic energy. All forms of electromagnetic energy travel in waves. The length of the wave, or wavelength, is what determines whether the wave is visible light or one of the other forms. The wavelength is measured by finding the distance from the top of one wave to the top of the next wave. Think about it in terms of measuring the distance from the top of one ocean wave to the top of the next, although the wavelength of visible light is much shorter than ocean waves. The wavelength of visible light is between 400-700 nanometers, which is about the size of a bacteria or virus. Blue has the shortest wavelengths, whereas red has the longest wavelengths. Mixing two or more of the wavelengths results in a blending of the two colors. White light is made up of all of the colors within the visible light spectrum.  The sun emits white light, which travels in a straight line through space, which is a vacuum. When the light hits the gas, dust, and water particles in the atmosphere, it is reflected, scattered, or absorbed by the particles. Blue light, which has the shortest wavelength, is scattered when it hits the tiny molecules of gas. The blue light is scattered in all directions, which is why the sky appears blue when we look at it on a cloudless afternoon. During a sunset or sunrise, the sun is on or near the horizon. The light has to travel a longer distance through the atmosphere. Almost all of the blue light is scattered and does not make it to our eyes. Colors that have the longest wavelengths are scattered the least, and therefore make it the furthest. Red has the longest wavelength and is scattered by larger particles in the air such as dust, pollution and water molecules. This is why the sun appears red and why the sky and surrounding clouds appear red as well. The blue light, however, is scattered and re-scattered so many times that most of it doesn't make it to our eyes. 


Last Updated: 9/18/13