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Q & A: Color vs Heat

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Most recent answer: 02/08/2011
Q:
I am doing a science project on color vs. heat absorption. I was wondering how does color absorb the light and turns it into heat.
- Yao (age 16)
St. Scholastica Academy, Chicago, IL, USA
A:
What we perceive as color is really various frequencies of light. Visible light is a form of electromagnetic radiation, along with Radio, X-rays, and the like. Often we describe the wavelength of the light, which is related to the frequency by:

Speed of Light= Frequency times Wavelength.
In increasing order of frequency, we have: IR-R-O-Y-G-B-UV.
IR (infrared) and UV (ultraviolet) aren't visible. Red light (R) has the lowest frequency of the visible spectrum. Blue Light (B) has the highest frequency. White light is the combination of all the frequencies of light.

 The color something reflects is the color you see. So, if you wear a black shirt on a hot day, you'd absorb all the frequencies of light (every color). According to the first law of thermodynamics, energy can neither be created nor destroyed. So when light hits an object, the energy has to go somewhere. Energy can change forms. So the light energy changes into heat energy, heating the air molecules in the Earth's atmosphere and heating the surface of whatever the color is on, like your shirt.

How hot the colored surface gets depends on what color it is because that describes what frequency(s) light it absorbs. The best way to get something hot is to make it absorb all of the frequencies (in other words, paint it black).

the effects of light on objects (including our skin) depend not only on how much total energy is coming in but also on what size packets of energy are absorbed in single steps. It turns out that the energy per packet (called a quantum of energy) is proportional to the frequency. Since UV has high frequency, its quanta are big, and the energy per quantum is able to do a lot of damage to molecules. Thus UV causes sunburn by damaging various molecules in your skin. The same amount of heat coming in as visible light doesn't do that damage.

Jason (w. some changes by Mike W.)

(published on 10/22/2007)

Follow-Up #1: heat to light

Q:
Can heat turn back into light?
- Crews McCulloch (age 78)
Graham, Texas, USA
A:
Sure, although here we're not using the word 'heat' quite the way purists would like. Turn on an electric heater. Look at the coils. You can see the light coming out.

Mike W.

(published on 10/05/2009)

Follow-Up #2: Which color absorbs more light?

Q:
Hi! Thanks for the info. Going back to the 'light vs heat absorption' topic, now i know that black absorbs light energy at a higher rate than other colors because it absorbs the whole range of wavelengths in white light. But if we compare two colors(i.e. blue vs red), which one, then, is able to absorb light energy at a higher rate than the other?
- John (age 23)
Singapore
A:
There's no general rule for that. For one thing, it depends on what colors of light are present to be absorbed. So let's pick some standard mix- say white sunlight. Even then, the answer is not definite. You can have different blue pigments which absorb somewhat different parts of the spectrum, and the same for red, etc. No one name of color is consistently a better broad-band absorber.

Here's another twist. We determine colors not by sensing the full spectrum of the light reaching our eyes but only by the ratio of how much is absorbed by three different pigments in our eyes. So you can have different paint pigments which look exactly the same to us in white light yet which absorb somewhat different components of that light. So we really can't make a general rule about which color absorbs the most light. Obviously, however, darker shades are better absorbers than lighter shades.

Mike W.

(published on 02/08/2011)

Follow-up on this answer.