Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Transfering Heat
Name:  Katy M.
Status:  student
Age:  20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Can you please settle a dispute about the effect of color on heat transfer IN THE DARK? I say that since dark colors absorb more light and that's what results in their increased heat, if they only way they are receiving heat is though means other than light, the color will make no difference. My BOYFRIEND, on the other hand, thinks that a baked potato will cook more quicky with the shiny side out, and that you'll sleep more warmly in a sleeping bag with the shiny lining, and that this is why house insulation has a shiny coating on it. (of course not only is the "color makes a difference in the dark" logic wrong in my opinion, but it's ALSO backwards because in the light it'd be the dull, not the shiny, objects that would absorb more heat, right? In addition to shiny objects in the dark, could you also address different colored objects in the dark?


Replies:
Katy,

There are three basic methods for transferring heat from one object to another: conduction, convection, radiation. Conduction is the method we are most familiar with. Conduction is a hot object touching a cold object. Convection is moving liquids and gases carrying heat from one location to another. An example of this is air circulating through ducts and vents. Neither of these has anything to do with color.

Radiation is radio waves, microwaves, light waves , x-rays, etc. carrying heat energy from one surface to another. Visible light depends on color. In the dark, there is no visible light. Infrared light is the most common heat transfer radiation in the dark. Some material can easily absorb infrared radiation, and some cannot. Again, it is not based on color.

What does affect it is "shininess". A shiny surface is very smooth. It is easier for any radiation to reflect from a smooth surface. Shiny metallic surfaces tend to be smoothest and best reflective. This is not due to color: a smooth metallic surface has no little grooves or holes in which to trap extra waves. Color as we define it only relates to visible light.

One comment about a baked potato: in an oven very little heat transfer is due to radiation. The majority results from hot air coming in contact with the potato, or with the foil around it. I do not expect the shininess of the foil will make a big difference. It is important that foil make very good contact with the potato. Air pockets between the foil and potato act as insulating zones, slowing the heat transfer.

Dr. Ken Mellendorf


Katy,

We have a mixture of concepts involved in your question. Speaking of light absorption: You are correct, darker colors absorb light (and heat) better than lighter colors. However, sleeping inside a sleeping bag with a shiny lining will cause more of your body heat to be reflected back to you. That is why thermos bottles are made with a silver lining. The shiny surface reflects incoming heat to keep the cool contents of the bottle cool just as it can reflect outgoing heat to keep warm contents warm.

Regarding foil-wrapped baked potatoes (never cooked in a microwave oven -- unless you want to damage the oven): When heated in a conventional oven, the foil around the potato reflects incoming heat. However, the foil itself heats up and ultimately transfers heat to the potato, thus allowing it to cook. Once cooked and removed from the oven, the foil reflects heat back to the potato so that it stays warm longer than if it had been immediately unwrapped.

Try this experiment: Wrap a potato in plastic wrap and cook it in a microwave oven. It works like a champ and is faster than cooking the potato in a conventional oven. It is done that way in some restaurants because it's quick and the plastic wrap keeps the moisture in the potato. However, no diner wants to see his/her potato delivered in a messy, unattractive piece of limp plastic that's a pain to remove. So the cook simply removes the plastic wrap and replaces it with foil before the server delivers it to the diner. While snug in the foil, the potato remains warm.

BTW, you can cook corn on the cob quickly and very nicely by wrapping it in plastic wrap and using the microwave oven.

Regards,
ProfHoff


I am still having trouble locating the shiny side of the baked potato . :)

As for insulation, the "shiny" foil is a vapor barrier which has purposes other than insulating; the vapor barrier is placed towrd the warm part of the area being insulated .

My suspicion is that the insulating value of the materials you mention have more/most to do with the insulating value of the materials themselves and less/little to do with a shiny surface.

Within a laboratory setting, one could probably measure minute differences in heat radiation from shiny versus dull surfaces of the same materials, but in a real environment setting, I dont personally believe one would notice a difference in feeling of 'warmth' in a shiny vs. non-shiny sleeping bag material, assuming the insulation materials were of equal structure and weight.

Thanks for using NEWTON!

Ric Rupnik


In total darkness [as the term is commonly used] there is no light, so NO color. On the other hand, there can be infrared radiation, to which the eye in not sensitive. Home insulation has aluminum foil back for two reasons: to reflect infrared [heat] radiation, and to provide a moisture barrier. The same reasoning applies to sleeping bags.

In the case of aluminum foil around a baking potato, I suspect that the major effect is the moisture barrier the aluminum foil provides so that the steam cooks the potato more rapidly if it can't escape.

Vince Calder


"In the dark" means no visible light, but visible light is only a snippet of the full spectrum of electromagnetic radiation (infrared, microwaves, ultraviolet, etc.), and the color of an object in visible light says nothing about whether the object will absorb or reflect other wavelengths. Aside from this, heat transfer doesn't have to involve radiation. In heat transfer by direct contact, of course, color doesn't matter at all.

Tim Mooney



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory