Unless you’re a hermit, you’ve experienced the strength of the sun many times in life. It doesn’t even require long days at the beach or afternoons spent at outdoor events. Just standing outside on a bright, sunny day provides a glimpse of its power. The evidence is in the sweat that trickles and the red tone that rises on your bare arms, legs or face. In fact, our bodies are the sun’s best way to boast.
Whenever you spend a little too much time outside with exposed skin, you can count on some amount of sunburn. Sunburn is the result of overexposure to the sun’s ultraviolet radiation. It causes chemical changes in the skin, leading to alterations in color, among other physical changes.
For many, the reward of sun exposure is a nice tan. However, due to the chemical changes taking place, this can come at a price: overexposure damages the DNA, sometimes even causing skin cancer. In other words, there are risks and rewards when we spend time in the sun.
But what about for nonliving things?
Humans aren’t unique in their susceptibility to sun damage. Along with living creatures, inanimate objects undergo changes after sun exposure. And such changes include alterations in color.
One key ingredient: ultraviolet radiation
The sun is the primary source of ultraviolet radiation, a type of electromagnetic radiation that is present in sunlight. It is named as such because UV light is beyond the visible spectrum of light, which ends with violet, the highest frequency of visible light. Thus, “ultra”-violet.
There are three main types of ultraviolet light: UVA, UVB and UVC.
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At the top of Earth’s atmosphere, right on the edge of space, 10% of sunlight is made up of these types of UV light. By the time sunlight hits the ground, it is only 3% UV light, and of this light, over 95% is UVA, the rest UVB. This small amount of UV light is strong enough to cause cancer with overexposure; however, the intensity is not uniform.
According to the American Cancer Society, several factors impact the strength of rays when they hit the earth:
Time of day: UV rays are strongest between 10 am and 4 pm.
Season of the year: UV rays are stronger during spring and summer months. This is less of a factor near the equator.
Distance from the equator (latitude): UV exposure goes down as you get further from the equator.
Altitude: More UV rays reach the ground at higher elevations.
Cloud cover: The effect of clouds can vary. Sometimes cloud cover blocks some UV from the sun and lowers UV exposure, while some types of clouds can reflect UV and can increase UV exposure. What is important to know is that UV rays can get through, even on a cloudy day.
Reflection off surfaces: UV rays can bounce off surfaces like water, sand, snow, pavement, or grass, leading to an increase in UV exposure.
There are also factors that influence the amount of UV exposure a person receives, such as length of time exposed and type of protection (clothing, sunscreen).
When each of the above are combined, they affect how the sun changes the color of inanimate objects we see.
So how do we see color?
How the eye works
The human eye uses three cell types in the retina to distinguish color. Called cones, each cell type is most sensitive to a different wavelength of light: short wavelength, medium wavelength and long wavelength. Short cones can detect light wavelengths in the range of 400 to 500 nm, medium cones 450 to 630 nm and large cones 500 to 700 nm. These each cover a different, often overlapping segment of the range of visible light, and when stimulated, the cones send signals to your brain to process color.
Single or multiple cones may show high stimulation when exposed to visible light, but it all depends on the particular wavelength. For instance, a short cone may react sharply to 450 nm wavelengths while the same light barely registers with medium cones. At the same time, each cone type has individual peak sensitivity (i.e., a strongest reaction) to specific wavelengths, and the overlap in detectible ranges allows two cones to react with the same strength to the same wavelength. For example, both the medium and long cones may respond equally as strong to wavelengths of 575 nm, even though they each react most strongly to other wavelengths. Combined with current knowledge, these facts help complicate what we thought we knew about sight.
If you’re like me, you’re probably most familiar with previous thoughts on sight, where scientists believed that short, medium, and long cones corresponded to the colors blue, green, and red, respectively. However, we now know that cones do not correspond to specific color detection, and that they each detect other colors as well. Interestingly, peak sensitivity in each cone can differ from person to person, even among those with normal color vision. In other words, your short cones may react most strongly to a 425 nm wavelength, whereas mine react most strongly to 440nm, both of which are different hues!
How objects show their color
Despite changes in our thinking about vision, one thing is still certain: while each person’s cones may react differently to the same wavelength, all of our cones need to detect reflected light for us to distinguish color. For example, imagine you an apple. The red color you see is based on the wavelengths that are bouncing off, rather than being absorbed, by the fruit. In other words, the apple skin doesn’t have red in it; it is reflecting the color red as light bounces off of it, due to chemicals in the peal.
I know what you’re thinking. Black and white seem different, but the same principle applies.
We see white when all of the colors are reflected. This is why the sun at noon looks white: all of the colors are reaching the eye. But why does it look yellow, orange or red at other times of the day? As the sun’s position changes in the sky, shorter wavelengths, such as blues and greens, get scattered in the atmosphere, leaving longer wavelengths, the reds and oranges, to reach your eye.
Black, on the other hand, is the lack of color. When you look at a black object, like a shiny new tire or the outer frame of your TV, there is no light reflected out because it is all absorbed by the object. And when there are no reflected wavelengths, there is no color.
Putting it all together: it’s in the chemistry
Although not the only possible culprit, UV radiation plays a significant part in the breakdown of the colors we see in outdoor objects. Unlike skin, an inanimate object doesn’t have DNA to destroy, but it is made of chemicals that can degrade. These chemicals, such as dyes, are subject to the same factors that cause sunburn, and thus the strength of UV light and the amount of exposure objects receive cause chemical changes that alter, among other things, color.
Remember the apple skin mentioned before? The chemicals in its skin do not allow the absorption of certain wavelengths, instead reflecting them. When it comes to objects such as an outdoor table umbrella, the same idea applies.
If you consistently leave a blue umbrella outdoors, the compounds and molecules that reflect blue wavelengths will be exposed to varying strengths of UV light, depending on the weather, location, proximity to reflective surfaces, etc. Over time, the energy from the constant battering of UV light wears down the chemical bonds between the molecules, eventually breaking them and leading to gradual fading. This is called photodegradation.
In other words, the color fades because the molecular bonds changed and the object can no longer absorb and/or reflect certain wavelengths on the visible spectrum. Objects may even eventually turn completely white because the wavelengths reflected have changed to that beyond our visible spectrum.
So then why do we get darker in the sun?
One thing we have that objects don’t is melanocytes. These cells exist in the lower layer of the epidermis, and when UVA rays are absorbed into the skin, they activate the melanocytes, which then produce melanin, the skin’s natural protector.
Melanin is a pigment that darkens the skin beyond your natural skin tone to protect from the harmful effects of sunlight. It absorbs light and disperses UV radiation, but as anyone whose tan has faded knows, the effect is only temporary. Every 28-30 days, the newly darkened cells make their way up to the surface and are shed, just like all skin cells.
Is there any way to protect the color of outdoor objects?
It’s not likely that you can completely prevent color fading from sunlight exposure, but you may be able to slow it by making careful choices. For instance, some dyes and other chemicals are stronger against UV; since red dyes appear to fade most quickly, it may be best to avoid the color. There are also spray-on chemicals available that claim to block UV and prevent fading.