Thread: Oh to finally have a roof over my poor head - new pictures from 06/03

Thanks Tin, when I said "It was easier to agree then to fight about it" I meant with the locals, not you.

I read somewhere that red roof tiles are just slightly less hear reflective than white ones - can't find the source now, of course but it did make some sense at the time, heat being in the infra red scale of the spectrum. And terracotta tiles being - red.

But my source didn't mention a 300% difference. I did broach the subject of using white tiles with my architect but he was so offended by the mere idea "You would be alone on Bali! Can not do!" I didn't have the heart to do it although I did look into it a little more in the form of painting some red tiles with a good covering of white paint which I left out to weather for a couple of months. The results where sort of pinkish... very "Gay" as my daughter said.

You got the idea, but you don't have all the "pieces of the puzzle." What I will try to explain, I am sure you know most of it already, but the WHOLE picture must be considered.


-The sun emits a whole spectrum of electromagnetic radiations, from high frequency x-ray (short wave lengths) to extreme infrared (long wave lengths).

-Thia solar spectrum is closed to a "black body" spectrum of 5,785 degrees Kelvin (5,512 degrees C), which peaks in the visible range of the spectrum, at about 4.750 (.blue), at the top of Earth's atmosphere, and more in the greenish, at about 5,250 Angstroms, at sea level.

-These radiations are made up photons, each carrying a certain amount of energy, the higher the photon's frequency, the more energy it carries. Therefore, a violet photon has more energy than a red photon.

- An object has a given color given by the wave length of the photons that it reflects most: a red object appears red because it reflects mostly "red" photons, and absorbs the rest, a blue object reflects the "blue" photons, and appears blue, etc. A black object does not reflect any of the visible spectrum wave lengths and therefore appears black. Thus, if you illuminate your red tiles with, for example, only blue light, they would appear black, because there are no "red" photons to be reflected.

So, the red tiles will reflect the red wave length ONLY (but NOT the infra-red photons). They will absorb all the other photons of different wave length, and the shorter the wave length, the more energy each photon carries, and this energy will be transferred to the tiles.

Energy is of course CONSERVED (one of the most basic physics principle). So this energy transferred to the tiles contributes to shaking the s**t out of the molecules which make up the material of the tiles. But molecules vibrate at low frequencies (long wave lengths) and will re-emit this energy at longer wavelength, in the infra-red, which is called in every day life, "heat."

I hope this explanation is clear (the exam will be closed book, next week)

It all comes down simply, as mentioned before, to the albedo of a given object: the darker the color, the more it absorbs the solar radiations, the easier it heats up.

Thanks Tin,

One further question: my, admitted aged physics knowledge reminded me that the light reflected is not only that frequency, be it red, blue or white, but contains all lower frequencies spectrum too. For instance: white objects will contain all visible frequencies but red objects will only reflect and contain frequencies lower in the spectrum?

Is your area of expertise physics? if so which specific teaching?

Markit

You are correct.

VERY simply, photons of all frequencies impinging on an opaque surface interact with the molecules of that surface. The photon will either be reflected "specularly" (like a mirror), scattered (reflected with diffuse scattering, i.e. in all directions), or absorbed – or some combination of these three possibilities. Opaque objects, like the tiles, which have rough surfaces that do not reflect, have their color determined by which wavelengths of light they scatter more, due to there molecular composition.

A very small amount, maybe 1 in 10 – 100 million photons (depending on the scattering material) of the scattered light will be scattered by an excitation (this is called the Raman effect). In that case, its energy will be reduced, that is to say that, like you said, its frequency will be reduced. In the case of the red tiles being illuminated by a red laser, a VERY small amount of the scattered light will also contain a VERY small amount of "redder" photons, and one could really see it (Raman Spectroscopy). In the case of solar illumination, what is called "red" will be the sum of the red and redder photons, and we won't know the difference.

I hope my explanation is not too garbled.

Your other question. 40 years of research in the field of nuclear physics, astrophysics (x-ray astronomy), and applied x-ray physics. Taught several years at Caltech, (2), Ohio State U. (2), and Northeastern U. (part time, 6)

Originally Posted by mat

I like the Al foil, not only good for reflecting heat...

Brings back old memories. I remember when I was teaching, there was ALWAYS a student in each class who came to the lecture just to sleep...

Ok Tin I got a question that's been buggin me for years and years and it is about as far off-topic as you can get - but it's my topic so I can do this I hope?

We (us plebs) are always being told that (voice of Carl Sagan "billions and billions and billions").

"These pictures from our fantastic Mega-buster, Super Acme Pretty Neat Neutron Telescope show the universe as it was 14 billion years ago back at the time of the big-bang."

Now if I follow this correctly we (the universe, my baseball card collection and all videos of Tracy Lords) at the time of the big bang (or as close as you can get) were about the size of a pin head (I know, no pins then either).

Bang! Everything (the universe, my baseball card collection and all Tracy Lords videos too) start to expand at what ever rate matter does in that situation - but! slower than the speed of light!!! Right?

My question is: if we have been expanding at a rate slower than the speed of light how can we get pictures made from any radiation that travels on or about the speed of light which passed us (tracy lords, cards, etc.) by about 1 trillionth of a second after the big bang. And has been ahead of us expanding for the last 14 billion years?

It seems to me it would be about like trying to use your camera flash to take a picture of your camera flash attachment - if that makes any sense?

Thanks for any info
Markit