International Day of Light Pre-celebration Edition: Some light questioning about…light

The electromagnetic spectrum, an assortment of energy wiggling throughout space and time, is overwhelmingly underappreciated in our lives. There is no combination of existence that could happen without it. To celebrate the role that light plays in our lives, our ecosystem, and the operation of the universe, UNESCO declared March 16th as the International Day of Light, a day to celebrate “ vital role of light and light-based technologies in science, culture and art, education and sustainable development”. Across the world, citizens are planning events to celebrate light in all of its forms.

Everywhere around us are waves, whether we can see them or not. Out of all the waves traveling through space, we can only see a fraction. On earth, we’re only seeing ~44% of the radiation coming from the sun! Even so, scientists estimate that we can differentiate up to 10 million colors in the visible spectrum.

Everything we can see in this universe is detected through waves, with wavelengths far smaller than the cells circulating through our body, thinner than the hairs on our head! The ways in which these waves reflect, refract, bend, and filter affect how we see the world.

After going down a Wikipedia rabbit hole about light, I came to a conclusion: I know nothing about light. Turns out, most people don’t. In the spirit of learning and continuing education, I’m here today to answer your questions about light, how it works, how it interacts with organisms, and society!

Why are colors different? And how can we tell?

Different wavelengths of light can create different colors, but what exactly does that mean? How do our eyes actually process this information? When we look at light, we are looking at how waves of photons are reflected and absorbed from a surface. This means that your favorite neon jacket isn’t actually yellow, it’s just that the surface is reflecting yellow light back at you. In a way, your jacket isn’t yellow, it’s actually everything but.

When the reflected light enters your cornea, it’s bent through the pupil, which contains thousands of nerve cells called rods and cones. Three types of cones exists, and each are sensitive to different wavelengths of light. When a specific wavelength hits these cones, the corresponding cone sends a signal to the brain telling you what wavelength of light you saw, and how strong the signal was.

What causes the sunrise/sunset to have different colors?

If you stare at the LA skyline at sunset, you’ll see a beautiful expanse of fiery orange surrounding the pollution-induced haze of the landscape. But at the same time and same longitude, the sky above western Canada could be a splendid purple. The color you see during sunset is always dependent on tiny, almost invisible particles in the atmosphere. When light hits these little molecules, they can scatter, causing light to travel in many different directions. When the sun is low on the horizon, light is passing through more of the atmosphere, meaning there are more opportunities to bounce off of particles. Light with smaller wavelengths (think blues and purples) are scattered the easiest, flying in many directions, while longer wavelengths (reds and oranges) can travel a fairly straight path. Ultimately, light will be filtered towards longer wavelengths. In polluted areas like LA, a high volume of atmospheric particles actually creates beautiful sunsets!

So any insta-worthy picture of a sunset can’t be #nofilter, because the atmosphere is actually a filter! A great one I’ll add.

Why can’t we see other wavelengths of light?

If humans could see into the infrared and ultraviolet domains, it would be really cool. We could tell if something is hot just by looking at it, see all kinds of energy being emitted, and have a lot of bragging rights. Unfortunately, we can’t. When ultraviolet light hits our eyes, it has so much energy that it ends up ionizing the materials in our cones that would send electrical signals to our brain. You can’t send a signal if your transmitter has just been destroyed. For long wavelengths like infrared, the energy transmitted isn’t high enough to trigger an electrical signal.

Technically, under the right conditions, we can see infrared photons. Actually, some people can see UV light, through a condition called aphakia. People with a genetic mutation have one extra type of cone cell, allowing them to see roughly 100 million colors.

Some animals can perceive a larger array of light then we can, for example, snakes can see infrared light! One of the many reasons why snakes are impressive creatures.

What makes things shiny?

Babies like shiny things, birds like shiny things, adult humans like shiny things–establishing a reason why we like such things is beyond the scope of my investigation, but why exactly do they shine?

The shininess of an object has to do with how it reflects light, but unlike color, it doesn’t matter what the wavelength is, it all has to do with electrons. In all atoms, clouds of electrons are orbiting the nucleus. When the electrons are hit by light, the electromagnetic field changes in the electron cloud, polarizing the cloud. Through this process, electromagnetic waves are propagating through the surface of the material. The easier they are able to propagate–that is–the smoother your surface is, the more shine you will see. That’s why freshly polished silverware is shinier than rusty utensils, they are smooth!

Can rainbow really be your favorite color?

There you have it folks. Have more questions about light? Comment below and we’ll try to answer them!

– Lissie Connors

What happens when several thousand distinguished physicists, researchers, and students descend on the nation’s gambling capital for a conference? The answer is “a bad week for the casino”—but you’d never guess why.
Lexie and Xavier, from Orlando, FL want to know:
“What’s going on in this video? Our science teacher claims that the pain comes from a small electrical shock, but we believe that this is due to the absorption of light. Please help us resolve this dispute!”
Even though it’s been a warm couple of months already, it’s officially summer. A delicious, science-filled way to beat the heat? Making homemade ice cream.

(We’ve since updated this article to include the science behind vegan ice cream. To learn more about ice cream science, check out The Science of Ice Cream, Redux)

Over at Physics@Home there’s an easy recipe for homemade ice cream. But what kind of milk should you use to make ice cream? And do you really need to chill the ice cream base before making it? Why do ice cream recipes always call for salt on ice?

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