Why is the Sunset Red?

Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

At the end of the day as the sun gets lower and lower on the horizon, the sun and the sky around it turn astonishing shades of red and orange, resulting in a beautiful sunset. In this lesson, we will explain exactly why that happens, and you will never look at a sunset the same way again!

Sunsets and the Color of the Sky

If you've ever watched a sunset, you have no doubt seen the color of the sky slowly change from blue to shades of red, yellow, and orange.

Blackbirds fly across a field as the sun sets in Kansas
A red sunset in Kansas

Why does this happen? Where did all the blue light go? To understand why the sky appears a reddish shade at sunset, it is important to know about Earth's atmosphere and how light waves interact with it.

Composition of the Atmosphere

The Earth is surrounded by an atmosphere that extends for many miles above the surface. The atmosphere has several layers, which get less dense and colder as the distance from the surface increases.

Earth's atmosphere is made up of many gas molecules. Nitrogen and oxygen together make up most of the atmospheric gases, with nitrogen making up about 78% of the total gas in the atmosphere, and oxygen comprising about 21%. The remaining 1% contains carbon dioxide, water vapor, argon, and a few other trace gases.

As light from the sun passes through the atmosphere, it interacts with these atmospheric gas molecules and this is what gives the sky its color. Before we can understand how that works, though, we need to first learn about light waves.

The Composition of Visible Light

Light from the sun appears colorless, but it actually contains ALL the colors of light mixed together. You can see all the colors in sunlight if you let light pass through a prism. Each color of light bends a little differently as it passes from the air into the glass and back again on the other side; when light comes out of the prism, it is separated into many colors. This is the same thing that happens when you see a rainbow. Light passes through water droplets suspended in the air that act like tiny prisms, separating sunlight into many colors and creating a beautiful rainbow.

White light is composed of all colors of light. When light passes through a prism, each color bends to a different degree which allows you to see all the colors that are present.
Visible light spectrum

So, we now know that sunlight is made up of many colors, but have you ever thought about what light really is? This turns out to be a really difficult question that has fascinated some of the most influential scientists throughout history, including Isaac Newton and Albert Einstein. Today we know that light has properties that make it seem like both a wave and a particle. Light travels through space as an electromagnetic wave, and when it interacts with other objects, it acts like it is made up of tiny particles called photons. Different colors of light waves have different wavelengths, with blue and purple light having the shortest wavelength (400-500 nm) and red light the longest wavelength (620-750 nm).

Different colors of light have different wavelengths
wavelengths of light

When light waves hit other objects, some of the photons can be absorbed and then re-emitted in a different direction. This is known as Rayleigh scattering and is very important for giving the sky its characteristic color, both during the day and at sunrise and sunset.

Rayleigh Scattering

As light passes through Earth's atmosphere, it runs into the many gas molecules floating around. When this happens, some photons of light are absorbed and re-emitted in different directions. Because of the difference in wavelength between blue and red light, it turns out that light near the blue end of the spectrum is scattered much more than light on the red end of the spectrum.

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