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Name: Alexa T.
Status: student
Age: N/A
Location: N/A
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Date: N/A

In my science experiment I painted identical jars different colors and left one clear. They were filled with equal amounts of water, then placed two at a time in front of a sunlamp. The temperature of the water after an hour was cooler in the white jar and warmer in the black as I expected, but why was the colorless jar just as warm as the black?

Hi Alexa-

I think I know what is happening in your lamp experiment. Your sunlamp was an incandescent floodlight bulb, right? (The kind of bulb with a bright tungsten-metal wire inside making the light... check with your teacher if not sure.)

It is too bad, but so far man-made light-bulbs just are not the same as sunlight for these very reasonable experiments.

In addition to making visible light, Incandescent light-bulbs make lots of invisible "infra-red" light rays, which you cannot see but they carry heat energy just like visible light. in fact, incandescent bulbs are pretty weak (inefficient): They use 100 Watts of electric power to:
project maybe 3 watts of visible light rays (not much!),
project 10-20 watts of InfraRed light rays,
and about 80 watts of heat which just makes glass of the bulb hot.

(No wonder there is publicity that we should use fluorescent bulbs, which make 3-4 times more visible light for a given 100 Watts.)

So there is maybe 5 times as much InfraRed energy as visible! (for this kind of bulb).

I think many of the infra-red rays bounce off the white paint, much like visible light does. All the infra-red gets absorbed in the black paint, exactly like visible. As for clear water and glass:
- all the visible rays go through, making no heat (also no problem)
- at least half of the InfraRed rays are absorbed in the water. (!)

It turns out that water, though very clear for visible light, is pretty dark for InfraRed light. Some scientists actually use thick tanks of clear water, just like your jar, as a special color-filter to take IR rays out of any bright white light.

Suppose you put lots of dark food-coloring in the water. For InfraRed light, that is what water looks like even when you do not add any coloring. I think if you make a jar of darkened water it will always get warm, just like the jar with black paint.

You see, just as there are many radio channels and your radio cannot hear them all, there are many frequencies of light, and our eyes only see a certain range we call "visible" light. This range is from 400 to 700 nanometers. 400 is the blue end of the range. 700 is red end. Past 700 is 'infra-red" Some common video cameras see IR (InfraRed) out to 1000 nanometers, called "near-InfraRed". I think water is still clear there. But the incandescent bulb makes radiation all the way out past 10,000 nanometers ("far infra-red"), very long slow waves. It makes them quite strongly, and the glass bulb lets some of them out. Past 1500 nanometers , called "mid-Infra-Red", the water in the jar is dark and absorbs these rays and gets warm.

My advice is to find a good south window with direct sun and repeat your experiment there, using sunlight, not bulbs. The sun makes mostly visible light and much less infra-red, so your light-absorption experiment will work much more like you expect and like what you see, if you use pure sunlight.

If you make a jar covered in front with aluminum foil, that will stay cooler than the white-painted jar, no matter sunlight or light-bulb. Metal foil reflects all InfraRed just as well as it reflects visible light. White paint usually absorbs a some of the IR, and you are never sure how much.

If your jars are on a broad counter top that gets hot in the sunlight, it might make sense to cover the counter-top with foil to stop that heating. This way temperature rise in your jar is due to absorption, and not to warm air rising up from the counter top. It may also nearly double the amount of sunlight shining on your jar, which should help make more obvious results.

Sorry if I made this letter too long. Please show it to your teacher, too.

Jim Swenson

Alexa T.,

Black paint absorbs visible light, gets warm, and then heats the water a little bit. White paint reflects visible light. No heating occurs from visible light. For the clear jar, the light actually reaches the water. Some of the light passes through the water, but the water directly absorbs some as well. Energy can get to from the sunlamp to the water for the black jar and for the clear jar. Energy cannot reach the water for the white jar.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

If the black jar heats more quickly than the white, that is an indication that the black paint is absorbing more radiation than the white paint. The black paint absorbs the radiation, warms up, and then transfers heat by conduction to the glass and water. However, the water itself can absorb radiation too. In the unpainted jar, the water and jar are absorbing heat directly (no paint), and therefore their temperature rises.

Even more complicated, just because something is black in the visible range of the spectrum does not mean it always/must absorb more infrared radiation. All objects have their own characteristic absorption spectrum for radiation, and you could find coatings that look black yet reflect infrared radiation. The same thing is true for 'clear' materials like water and glass. Even though they do not absorb visible light readily, it turns out water is very active in the infrared spectrum. So water is likely doing most of the infrared absorption.

If both the painted black jar and the unpainted jar are absorbing nearly the same amount of radiation, then that explains why the temperatures are nearly equal.

I would guess that if you performed the experiment more carefully, you would (or could) find a temperature difference between the unpainted and painted jars. You could also find different black paints that absorb radiation differently. The key here is 'absorbance' -- the amount of radiation that a material absorbs. Just like objects can have different colors (they absorb visible light differently), so can the absorb other radiation as well.

Burr Zimmerman

The white jar reflected light in the infrared, which is the major wavelength that is converted to heat, even though you can't "see" it with your eye. Where the surprise comes is that the water in the colorless jar still absorbs the infrared radiation -- even though you can't "see" it. Your thermometer is not sensitive enough to distinguish between the difference between a "black" jar and a "clear" jar because the radiation involved is not visible radiation but infrared radiation.

You did not mention whether the jars were open or had a lid. They all should have a lid to prevent evaporation, which could decrease the temperature rise because of evaporation, which reduces the increase in temperature.

Vince Calder

because the light was not reflected off the glass as the white bottle we have a case of the light passing through the glass......and absorbtion ....great scince project to compare those two and graph data.

prof przekop

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