June 20, 2014

Physics for Instagram: How Sunsets Work

Jeff Vincent

Operations

This post is part of our Non Sequitur Fridays series, which will feature a different Wistia team member’s take on a non-Wistia-related topic each week. It’s like our "employee of the month" but less "of the month"-y. Jeff Vincent is director of customer happiness at Wistia. His last Non Sequitur was about how rainbows work.

If you, like me, peruse Instagram during the down moments of the day, you’re already pretty familiar with the most popular photo archetypes - selfies, airplane wings, food (burgers and chips & guac!) and my favorite of all… sunsets.

Sunsets (and their less Instagrammed sister, sunrises) are one of the most apparent and ritual examples of the sheer beauty of science. Light, coming from the sun, appears in brilliant colors to our eyes because of the awesomeness of physics.

So what makes sunrises and sunsets so amazing?

To describe how sunrises and sunsets work, we need to start with that universal question you annoyed your mom with as a kid, "why is the sky blue?"

If you’re anything like me, your mom kneeled down, patted your head, and then said, "scram kid, you bother me!" (kidding, my mother is a loving and wonderful individual). But she also didn’t know exactly how this worked. Let’s break it down.

As the light from the sun approaches the Earth, it comes into contact with the tiny particles in our atmosphere (mostly nitrogen and oxygen). These tiny molecules are really good at scattering the shorter wavelength, higher frequencies of light, especially blue and violet.

As the highest frequency of visible light, violet is actually scattered most, but our eyes pick up the blue much better. So next time someone asks "why is the sky blue?" You can respond "umm it’s violet, duh." (I am not responsible for what happens next.)

If you really want to nerd out, check out this phenomenon, called Rayleigh Scattering, named for Lord Rayleigh, who also must have been really annoyed by children asking him why the sky was blue.

The scattered light during the day is mostly violet and blue, which is why we see a blue sky. Why does this change to oranges and reds in the morning or at night?

When we are turned such that the sun appears at the visual horizon, the light has to travel through more atmosphere to reach our eyes. The particles still do their scattering thing, and more of the blue light is initially scattered. By the time it reaches us, the longer wavelength red, orange, and yellow light is all that’s left. This scattering means we see the incredible colors of the sunrise and sunset.

That blue light that was scattered as it passed through the column of atmosphere? It hits the eyes of someone at a different point on the Earth, in the middle of their day. So cool.

Now that you know the physics behind them, what are some cool things about sunsets?

Sunsets really are cooler at the beach. It’s not (just) because you’ve got a cold beer, your toes are in the sand, and there is some dude playing guitar (which might totally ruin the moment). It’s also because the salt and water particles floating around closer to the beach scatter even more of the light, which means more intense reds, oranges, and yellows. Cool!

White and yellow sunsets happen when the air is less scattered before hitting your eyes. This occurs most often in areas where there is less environmental interference (such as smog and dust) and when there is less atmosphere to travel through (like at higher altitudes).

One of the coolest parts of a sunset is how big the sun can appear. This is not because we are closer to the sun at sunset (thank god, I don’t want to fry). Instead, the Sun and Moon appear larger at the horizon because of our old friend refraction. As the Earth turns and the Sun crosses our visual horizon, the light from the bottom of the sun bends, creating the optical illusion of a fatter Sun. Super cool effect.

From a strictly light science standpoint, sunrises and sunsets are no different. That being said, they certainly can appear different in the moment, right?

What’s up with that?

There are two reasons why sunrises and sunsets look different. First, the hustle and bustle of the day creates more airborne particles, which cause more scattering and diffusion of the light. This makes sunset colors appear more intense than they are at sunrise. Second, as we are making that bleary-eyed run for early-morning coffee, our eyes are dark-adjusted; they are used to the conditions of darkness. This can make certain colors appear different than they would at sunset, when our eyes are light-adjusted.

Light sure is neat! To get even deeper into cool sunset stuff, check out the green flash phenomenon (hat tip to Dave for that).

I hope you’ll post an awesome picture of a sunrise or sunset in the comments - regardless of the science, I just love looking at them!

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