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April 27, 2016

Infrequently Asked Questions: Why do we feel warmer when we wear black?

The world is full of questions we all want answers to but are either too embarrassed, time-crunched or intimidated to actually ask. In the spirit of that shared experience, we've embarked on a journey to answer all of the questions that burn in the minds of Philadelphians — everything from universal curiosities (Why do disposable coffee cups still leak?) to Philly-specific musings (How does one clean the Liberty Bell?). 

It's that time of year: The weather heats up, the sun returns from vacation and suddenly that winter wardrobe's worth of black clothing feels a lot warmer than it did a month ago. Black shirt on your back, you're feeling a bit toastier than usual. Curious, we reached out to Jamie Kennedy, a researcher in Drexel University's Nanophotonics+ Lab, to get some straight answers.

Why do we feel so much warmer when wearing black on a sunny day?

I think the root of the question comes down to understanding what the difference is between light and heat. Essentially they're the same thing, both basically packets [of energy] — photons, right? And the difference between them is light has a shorter wavelength than heat. So if we talk about the sun, it provides us with ultraviolet light, or UV light, and the [visible] light, and the infrared light. Infrared is what we know as heat. So the vast majority of what comes down is visible light from the sun — it's part of the reason why the sky is blue, red and different colors during different times of the day. But when heat hits something — visible light hits something — its wavelengths slow down. And when wavelengths feel the vibration, it slows down and it turns into heat. 

So, say you’re sitting in a bar and sunlight comes in through the window. The window reflects the heat but absorbs the light and the light passes through the window and hits your face or hands. When it does that you feel warmth from the light vibrations slowing down and turning into different light, which is heat. 

So now you get into colors. No proton is considered fast or slow. They all travel at the same speed: the speed of light. However, they do vibrate at a certain frequency. This frequency has a wavelength, and this is what determines what kind of 'light' it is. So visible light has a wavelength in the spectrum of 400 to 700 nanometers. Ultraviolet comes before at a lower wavelength, and infrared — or heat — comes after it at a wavelength of 700 nm to 1 mm. It starts at violet and ends at red — I’m sure you’ve seen the rainbow spectrum before in science class. Every color we see is on this spectrum. Every object or medium absorbs and reflects light. So, if we perceive an object to be green, it is only because it is absorbing every other color on the spectrum and reflecting green. 

When you look at black, it’s known that black is not actually a spectrum of color because black absorbs all light. When you’re absorbing all this light it’s naturally turning into heat. When a medium is absorbing light it’s absorbing energy, and they need something to do with it, so it’s exerting heat. So not only is it absorbing all the light, but then it’s also emitting light, putting off as much of that as possible. That’s the main reason wearing black makes you feel hotter: It’s not only absorbing as much light as possible, which is naturally turned into heat because it slows down when it hits a medium, but it’s also going to emit heat as much as possible.

Does this mean it's not just black that will make you feel warmer?

If you look at a spectrum of colors from lightest to darkest, the darker colors you’d wear, the hotter you would feel. Darker colors naturally absorb more light.

Is this also why we’re led to believe we should wear lighter colors in spring and summer? Is there a connection there?

There definitely is. I think it also depends on the actual material you’re wearing. Because the more light that can pass through things is also not being turned into that, right? So if you’re wearing lighter material and walk into the sun, wearing something that’s almost transparent, light’s gonna actually not be absorbed as much as something you can’t see through. Wearing something light in color and material is definitely going to give you the lightest feeling in the summer for sure.

Any other little-known facts?

I think it’s cool we haven’t been able to create a purely black color. So, black is supposed to absorb all light, but theoretically, in science, we’ve only been able to create something called a ‘phantom black,’ which absorbs only 99.9 percent of light. So yeah, that’s as close as we’ve gotten to creating black. Even when you’re wearing a black shirt, it’s not actually absorbing every little bit of light. So a lighter shade of black will actually absorb less light. That’s a cool thing.

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