Ask A Scientist© Environmental Earth Science Archive

name         Logan H.
status       student
age          11
   Question -   I did my science project using different colored 100 watt
outdoor flood lights to melt ice. Just the flood light covering was
colored. I found out that the different colors put out different amounts
of heat even though they were all 100 watt. Then I learned that the 100
watt just meant how much energy they used. My question is why was the
heat of the different colored bulbs not in the same order as the visible
light spectrum colors. In my experiment green put out the least heat,
then red, then blue then yellow put out the most. How can I explain why
each color put out a different heat?
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Logan-
    I have thought about your experiment.  You might have discovered something about green dyes and IR heat-rays.
But it will take me a long time to explain:

   I think of incandescent bulbs as really funky primitive old beasties, like a candle flame.
So they will usually try to fool you in a science experiment about color.

Frankly, I think all your 100 Watt bulbs should put out about the same amount of heat, +/- 5%, if they are all really 100w.
So first, you have to measure how many watts each kind of bulb actually uses.  by using a electric watt meter or AC amp-meter.
They can sell you a "100W" bulb that uses only draws 80W, if they want to, and nobody can sue them.
So it probably happens a little bit.  I bet 95W-105W almost always happens.

Next, be aware that incandescent light bulbs are usually less than 3% efficient.
If they take 100W of electricity, they put out 3W of light (total of Red, Green, and Blue all together), and the remaining 97W all turns to heat somewhere.
"Somewhere" mostly means right inside the bulb, making it hot.
If you want to measure all heat coming out of the bulb, it will need to be surrounded by your apparatus, i.e., in a metal box with a window in the lamp direction.
Then measure how hot the box gets.

Even worse, some of the 97W does not turn into heat right away.  Maybe 10-20W of it turns into Infra-Red (IR) light first, which tries to shine out through the glass bulb, just like the Red, Green, and Blue, but you cannot see it.  You can feel it as warmth on
the back of your hand at 6 inches away, though.
When this infrared ("IR") light hits something, then it does one of the things normal light does: go through, reflect, or be absorbed and turn into heat.
The things it does not do are: help you see, power solar cells, expose camera film.  Actually the IR that is right next to Red does power solar cells.  Oh, well.
Anyway, it often sails up to your thermometer and adds to what you read.

The light-bulb makers have gotten pretty clever about coatings lately.
You can get "cold-light" bulbs where the coating stops the IR light from shining out.
You can get "heat-lamps" designed to throw forwards as much IR as possible, to heat food.  They look very red.
You can get "dichroic" colors which are very pure Green with no Red or Blue, but almost none of the IR heat is blocked.
And there are the normal-looking colored coatings which might stop a lot of IR, or might not.  Different amount for each color, I bet.
Is that what is confusing your experiment?

When they make a green coating, for example, they use chemicals which block red, and block blue, and do not block green.
They do not care whether the chemicals they chose block IR, and they have lots of different chemicals to choose from.
So it comes out almost random.
Unless you pay more to buy a bulb which says on the label that it blocks IR or heat; that effect you will notice.

Suppose you imagine that every color-dye chemical has only one absorption band, fairly broad.
Such as: a "blue" dye transmits blue, absorbs green, and absorbs red.  Will it be wide enough to absorb IR too? probably not.
I have noticed that most single-chemical dyes have an absorption band only about one color wide, or a little more.
A "green" dye must have two separate absorption bands, one to block Red and another to block Blue.  Probably it has two chemicals.
The one that blocks blue is quite likely to be wide enough to also block some UV, and the one that blocks Red might block some IR.
That tends to fit your observation that "green put out the least heat".
I wonder if red put out the most heat?  Nope, I am wrong.  But yellow is kind of like red, but the dye is thinner, likely to block less of everything.

I think color experiments are best done with sunlight and colored plastic or glass, because sunlight is at least 50% visible light.
Invisible light is harder to keep track of.

Jim Swenson
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