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I was reading an article on global warming and it said that the polar ice caps, because they are white, reflect a lot of the sun's radiation. The article also has a picture of some houses in England that are painted all white and supposedly these houses keep cooler in the summer and warmer in the winter.

I understand that the warmth that one feels from standing outside in the sun is due to infrared radiation from the sun. I also understand that the more infrared radiation an object reflects, the whiter it looks. At least, that is what the article I was reading impressed on me.

Anyways, here are some questions:

  1. If I expose an object to EM radiation only from the infrared spectrum, will it only reflect back infrared? Is this true for other types of EM radiation?

  2. Is it possible to make an object that looks white and absorbs a lot of infrared radiation?

  3. If an object reflects most of the EM radiation that it receives of a particular wavelength $\lambda$, will it also reflect most of the radiation it receives of wavelengths less than $\lambda$ (and absorb most of the radiation of wavelengths larger than $\lambda$)? Is this why objects that reflect most visible light (and hence look white) also reflect most infrared radiation (since infrared waves have a shorter wavelength)?

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2 Answers 2

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Wrong:

"since infrared waves have a shorter wavelength"

Infrared has longer wavelength than visible and visible longer wavelength than ultraviolet .

sun spectrum

White is a term for visible light mixed wavelengths. In the plot you can see that almost half of the sun's radiated energy arrives as visible light. The white buildings reflect this visible light which otherwise,impinging on the surfaces would be absorbed and turned into infrared by the interactions, adding to the arriving infrared.

What is absorbed and what is reflected depends on the chemical bonds of the surfaces, whether the incoming radiation can excite molecular states of the materials. Infrared is in frequencies/wavelengths of the black body radiation of bodies in the temperature ranges comfortable for the human body, so they easily raise the vibrational and rotational levels of solids and liquids and the kinetic energy of gasses.

black body 310K

37C curve seen here practically all in infrared, and lower temperatures more so.

Thus white paint will not reflect infrared as efficiently as visible, a large part of infrared will be absorbed as also some part of visible will scatter at the surface and degrade to infrared. Infrared can be reflected by metal mirrors, from the collective fields in metals . If you put aluminum foil in front of a heater you are sheltered from most of the heat which is reflected, but some of it is absorbed as can be seen by touching the foil.

If I expose an object to EM radiation only from the infrared spectrum, will it only reflect back infrared?

Yes, but most of it will be absorbed ( except by mirror metal surfaces) because the materials have the receptors for these wavelengths. This is due to the fact that larger wavelengths have photons with less energy which cannot excite higher energy levels.The energy of the photons goes as h*c/lamda where h is plancks constant, lamda the wavelength and c the velocity of light.

Is this true for other types of EM radiation?

No.Visible and ultraviolet by scatterings degrade their energy down to infrared frequencies, depending on the material.

Is it possible to make an object that looks white and absorbs a lot of infrared radiation?

Usually most of the infrared will be absorbed except by mirror metal surfaces.

If an object reflects most of the EM radiation that it receives of a particular wavelength λ, will it also reflect most of the radiation it receives of wavelengths less than λ (and absorb most of the radiation of wavelengths larger than λ)? Is this why objects that reflect most visible light (and hence look white) also reflect most infrared radiation (since infrared waves have a shorter wavelength)?

There is no such rule. It depends on the material and its chemical bonds.

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I don't know why I wrote that infrared has a shorter wavelength that visible light... Anyways, thanks for the answer. –  echoone Aug 30 '13 at 17:34

I understand that the warmth that one feels from standing outside in the sun is due to infrared radiation from the sun.

In part -- you can directly absorb some IR from the Sun. Most of it, though, would be visible light absorbed by you skin and clothes that warms up your skin and clothes, making you feel warmer.

I also understand that the more infrared radiation an object reflects, the whiter it looks. At least, that is what the article I was reading impressed on me.

No. "Whiteness" is purely visible light being reflected (at all wavelengths). Whatever gets absorbed by the object will eventually be re-radiated as IR (based on its temperature and how closely it approximates a "Black Body" perfect radiator). You can't see the IR, but you can feel it as warmth.

houses in England that are painted all white and supposedly these houses keep cooler in the summer and warmer in the winter.

Being painted white, they reflect the sun's rays (visible light) and therefore don't warm up as much as if they were painted a darker color. During a summer night, they won't radiate quite as much into the night sky, and therefore might not cool down as much as a darker house would. I doubt that they stay that much warmer in the winter -- during the day, they reflect away sunlight that would be better used to warm them, but during the night, they would probably radiate away less heat than a dark house, keeping them a bit warmer than they would be otherwise.

Note that I qualify radiative properties (emissivity) in the above. Being reflective at visible wavelengths does not automatically mean that an object is a poor radiator at IR wavelengths, and vice-versa. White paint could theoretically be very reflective at visible, and a very good emitter at IR. It all depends on its composition and physical characteristics.

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