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Physics of sunsets

As the sun rises, goes across the sky, and then sets, we see pretty much all the colors of the rainbow: red, orange, yellow, blue, indigo, dark blue/violet… but why is there no green?

Turns out that there are moments when one can see green light, but it is very rare. In  The Physics of Sunsets: Science blogs, Ethan Siegel we read:

When the Sun appears progressively lower and lower on the horizon, its light needs to pass through more and more of the atmosphere to reach our eyes.

You might not think of the atmosphere as being a very good prism, but when you pass through around 1000 miles of it just before the Sun dips below the horizon, it starts to add up.

The bluer wavelengths of light get scattered away, leaving only the reddest wavelengths that reach your eye.

As the sun drops towards the horizon, it progressively loses violets and blues,

then greens and yellows, and finally even the oranges, leaving only the reds behind.

You may not even realize it, but by time you’d see a sunset like the picture above,

the Sun has already technically set,

it’s only due to the fact that the atmosphere bends light that we’re still seeing it like this.

Image credit: R Nave of Hyperphysics.

Image credit: R Nave of Hyperphysics.

This is why, if you time a sunset, it will take longer than the expected 120 seconds to go from the moment it touches the horizon to the moment it dips below, even during the equinox at the equator, where it rises and sets as close to completely vertical to the horizon as possible. The Sun appears to linger due to the refraction of our atmosphere.

Also, despite its red appearance, there really still is blue and green light coming from the Sun, of course, while this is going on.

But these shorter (i.e., bluer) wavelengths refract slightly more than the lower frequency ones:

meaning that the reds come in at a different, shallower angle than the greens and blues, that come in at a slightly steeper angle.

Image R. Nave of Hyperphysics

Image R. Nave of Hyperphysics

Given a clear path to the horizon — such as over the ocean — this means that there’s a slight region of space just above the reddened Sun where only the shorter wavelength light is visible!

And when that happens, in addition to the normal color gradient that comes with a sunset, you can also get a small, separate region above the disk of the Sun that appears yellow, green, or even blue!

This optical phenomena is always most clearly visible over a flat area in pollution-free skies, and is known as the green flash. It occurs in many different stages, sometimes appearing at the limb of the Sun or just above it, but most it commonly appears just after the disk of the Sun has set, in a literal “flash” lasting just a few seconds, just barely above the horizon.

Although there’s a lot of green light in the Sun, the bluest wavelengths refract even more than the green ones do. In principle, you could get a “flash” of any wavelength — yellow, green, blue, or even violet — if the atmosphere cooperated. Although green and yellow flashes are the most common, under just the right atmospheric conditions, you can see even blue colors flashing at a high angle above the top of the Sun!

This applies to any very bright, white-light object that encounters our atmosphere as seen just barely above the horizon. So that means the Moon, which reflects sunlight back at us, should also exhibit a green flash under the right atmospheric conditions. And although I’ve never seen it with my own eyes, some diligent astrophotographers have captured the sight to share with us all.

Green Flash and Super Moon Laurent Laveder PixHeaven.net  http://apod.nasa.gov/apod/ap120510.html

Green Flash and Super Moon Laurent Laveder PixHeaven.net

Here is another excellent write up by Ethan Siegel: The Green Flash at Sunset

Colin Sullender writes:

Green flashes are an optical phenomena that sometimes occur right after sunset.

In the correct conditions, a green spot briefly becomes visible above the the sun as it disappears behind the horizon.

They occur because the atmosphere refracts and scatters the broad spectrum light of the sun differently depending on wavelength (this is why the sky is blue).

In general, refraction by air (atmospheric dispersion) is larger at shorter wavelengths, which means sunsets last slightly longer for violet/blue/green light than red light.

However, because the path length through the atmosphere is greatest when the sun is at the horizon, the effects of scattering are greatly increased (atmospheric extinction).

Since scattering affects shorter wavelengths the most, much of the violet/blue light is lost before reaching your eye, thus leaving only the green light briefly visible.

A perfect Green Flash Sunset, by Juan Guerra. https://www.youtube.com/watch?v=lwus2nqU0SY

A perfect Green Flash Sunset, by Juan Guerra.

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

HS-PS4-3. Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described by either a wave model or a particle model, and that for some situations involving resonance, interference, diffraction, refraction, or the photoelectric effect, one model is more useful than the other.

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)

Core Idea PS4
Waves and Their Applications in Technologies for Information Transfer
How are waves used to transfer energy and information?

When a wave passes an object that is small compared with its wavelength, the wave is not much affected; for this reason, some things are too small to see with visible light, which is a wave phenomenon with a limited range of wavelengths corresponding to each color. When a wave meets the surface between two different materials or conditions (e.g., air to water), part of the wave is reflected at that surface and another part continues on, but at a different speed. The change of speed of the wave when passing from one medium to another can cause the wave to change direction or refract. These wave properties are used in many applications (e.g., lenses, seismic probing of Earth).

The wavelength and frequency of a wave are related to one another by the speed of travel of the wave, which depends on the type of wave and the medium through which it is passing. The reflection, refraction, and transmission of waves at an interface between two media can be modeled on the basis of these properties.

All electromagnetic radiation travels through a vacuum at the same speed, called the speed of light. Its speed in any given medium depends on its wavelength and the properties of that medium. At the surface between two media, like any wave, light can be reflected, refracted (its path bent), or absorbed. What occurs depends on properties of the surface and the wavelength of the light.

How are instruments that transmit and detect waves used to extend human senses?

Understanding of waves and their interactions with matter has been used to design technologies and instruments that greatly extend the range of phenomena that can be investigated by science (e.g., telescopes, microscopes) and have many useful applications in the modern world.

Also see Benchmarks: American Association for the Advancement of Science



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