Sunrise, Sunset

  Sunset in Phoenix, Arizona!

Sunset in Phoenix, Arizona!

by Spencer Kost and Connor Murray


Twice a day, every day, some of the most spectacular and awe inspiring sights can be seen and all one needs to see it is to look up. That spectacle is, of course, the beautiful reds and oranges that spatter themselves across the sky, accompanying the rising and setting of the sun. This phenomenon has inspired humans for millennia, with such grand explanations as gods flying across the sky every day to light our days; however, the real explanation relies on the simple interactions between our Sun, our atmosphere, and our eyes.

To understand this phenomena, it is first crucial to define its distinctive characteristics. Observing the figure below, the main characteristic is the contrast of orange-red color closest to the horizon with the characteristic blue of the normal day's sky.

The difference in color will last for approximately one hour as the light from the sun progresses westerly across the earth's surface. This implies that the visible effects occur only when the light from the sun reaches our eyes at the fringes of its travel. Additionally, the colors seen and the intensity of them can vary from sunrise to sunrise, implying that environmental factors can have a significant impact on the observed properties.

Electromagnetic Waves

Sunrises and sunsets are mainly visual phenomena, defined by electromagnetic waves. An electromagnetic wave is created by the excitation of any charged particle, the most notable of which is the electron, one of the fundamental components of all atoms. All electromagnetic waves are an oscillation of two fundamental physical fields: electric and magnetic. These oscillations propagate at the speed of light (3.00 X 10^8 m/s in a vacuum) in all directions. The wave is then classified by its frequency (how many oscillations per second), and wavelength (the distance between peaks of each wave), with the product of the two being equal to the speed of light.

c = af

The energy transmitted by EM (Electromagnetic) waves is dependent on the frequency and wavelength, with waves with a higher frequency and thus shorter wavelength) carrying much more energy by this formula.

Energy = hf

h=planck's constant = 6.626x10 34m2kg/s

This creates a spectrum of different types of energy with the shortest wavelength producing high energy gamma waves and the longest producing low energy radio waves. The EM waves that humans are able to see are in the visible spectrum, which range from 380-750 nm, and are perceived by three "cones” on the back of our eyes, which individually perceive red, green and blue light.

  Artists impression of the light spectrum

Artists impression of the light spectrum

Electromagnetic waves can be produced naturally and continually, in systems such as those seen in our sun, or in a very specific manner by human made structures such as a microwave or radio tower.

From Sun to Atmosphere

The natural EM creation process, which concerns the phenomena of sunrises and sunsets, is mainly created due to heat. The heated objects emit electromagnetic radiation due to their atoms frequently changing energy states and in so doing produce EM waves. The exact nature of these waves differs from substance to substance. A piece of metal, when heated, will emit a red, visible spectrum glow, whereas glass heated to the same temperature will emit lower energy, Infrared radiation. The sun, which creates its energy and heat through nuclear fusion, gives off EM radiation in almost the entire spectrum of radiation, with the majority of the energy emitted as visible and infrared radiation. This radiation travels in all directions from the sun, with some of it traveling towards the Earth. Upon reaching the Earth's atmosphere (the envelope of gases that surround the Earth) the Electromagnetic waves interact with the atmosphere to create the visible effects familiar to us.

Electromagnetic Waves in the Atmosphere

When light travels through the atmosphere, it interacts with gas particles and is redirected, in a process called scattering. Scattering occurs when electromagnetic waves collide with particles in the air. There are three different types of scattering that occur in the atmosphere: Rayleigh scattering, Mie scattering, and nonselective scattering. Rayleigh scattering occurs when the particles are much smaller than the wavelength of the light it is interacting with, Mie scattering occurs when the particle is about the same size as the wavelength, and nonselective scattering occurs when the particle is much larger than the wavelength.

The type of scattering that accounts for the colors in the sky is Rayleigh scattering. Rayleigh scattering scatters mostly light with short wavelengths. This is because the intensity of the scattered light is inversely proportional to the wavelength of the incident light:

Intensity ~ +

                                                                                   Rayleigh Scattering when the sun is high in the sky

                                                                                 Rayleigh Scattering when the sun is high in the sky

Because of this relation, blue light, with its short wavelength is scattered best by small molecules in the atmosphere. This is also the reason that the sky appears blue and the sun appears yellow. As the light from the sun hits the atmosphere, the blue light is scattered in all directions. When you see the blue sky away from the sun, you are seeing this scattered blue light. The sun appears yellow because, by the time the light reaches you, most of the blue light has been scattered, leaving only yellows to be observed.

Sunrises and Sunsets

The same principle that applies to the blue sky and yellow sun also applies to why sunrises and sunsets appear red and orange. When the sun is low on the horizon, the light from the sun must travel through more atmosphere to reach you. When the light travels through more atmosphere, it is scattered more, and even more blue light is removed from the incident beam. By the time the beam reaches you, most of the short wavelength light has been scattered, and all that is left is the reds and oranges that you observe.

  Rayleigh Scattering When Sun is Low on the Horizon

Rayleigh Scattering When Sun is Low on the Horizon

There is the common perception that pollution helps enhance the colors of a sunset. This is not entirely true. Low-level pollution, such as smog, will actually subdue the colors of a sunset. Smog particles are roughly the size of the wavelength of visible light, meaning that smog produces Mie scattering. Mie scattering scatters all wavelengths of light with the same intensity, which results in smog turning sunsets more grayish, and muting the red colors.

However, pollutant particles that are in the stratosphere are small enough to produce Rayleigh scattering, and thus enhance the colors of a sunset. An extreme example of this type of pollution is after large volcanic eruptions. Volcanic dust from the eruption will float up into the stratosphere and scatter the light and sunset. There are so many of these particles in the atmosphere that the whole sky appears red, an effect known as afterglow.

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Spencer Kost and Connor Murray created this while students in the Department of Earth and Planetary Sciences at Northwestern University