How does light work? (A question for the scientists)

Post by **Hound** » Mon May 02, 2005 6:24 pm

Seriously...

Ok, the visual spectrum is the seven colors of the rainbow but what I'm wondering is does light have an actual color?

It doesn't right? It's just illumination right? It's all those colors blended to make just white light, which isn't a color so much as the presence of all colors.

Color is something we just see as light reflects off of objects or through objects, right?

I'm pretty sure I understand that much.

What I'm not sure of is things that give off light like stars but can be red or blue or orange. Where is the color coming from? What is ligght reflecting off of or through that gives it that color?

Many questions, I should've paid more attention in science class...

Ultimate Weapon · Post by **Ultimate Weapon** » Mon May 02, 2005 6:47 pm

Electric currents do not contain color. Though lightning and sparks give off what appears to be a blue white beam of energy, interpretted through the lens of the iris, as a wavelength frequency. Stars contain radiation and heat. This gives off light like fire. Now depending on the combustion of the material the flame can change it's color, like blue or yellow. Light I would say is electromagnetic radiation that can escape the confines of dark matter. By traveling at great speed, light can penetrate most gravities. The light we see has escaped and is running for it's life so to speak. To pass on information of it's existence. This is how humans hope to transport matter some day, through light and energy. The precise nature of light is one of the key studies to modern physics.

Halfshell · Post by **Halfshell** » Mon May 02, 2005 6:52 pm

Light can exist in several different colours, iirc. UV, Infra red, etc...

Different colours of light blend to form your "standard" light... you can see it broken down into its component parts (for lack of a better term) when it's refracted through a prism. A rainbow is a good example of this - the light passes through the water and splits across the spectrum.

"Pure" light doesn't split when prismed... I dimly remember being the only person in my class who managed to grasp the concept, so I didn't pay as much attention as the others were forced to.

Whack a beam of light into one side of a diamond (or some other prism) and watch the different colours shoot out of the different faces. Put the same prism into a "pure" beam of light (ultraviolet, etc) and all the beams that come out will be the same colour.

[/blah blah from memory blah]

Ultimate Weapon · Post by **Ultimate Weapon** » Mon May 02, 2005 6:56 pm

"Light is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. The three basic dimensions of light (and of all electromagnetic radiation) are:

* intensity (or brilliance or amplitude, perceived by humans as the brightness of the light),
* frequency (or wavelength, perceived by humans as the color of the light), and
* polarization (or angle of vibration and not perceivable by humans under ordinary circumstances)

Due to wave-particle duality, light simultaneously exhibits properties of both waves and particles."

http://en.wikipedia.org/wiki/Light

the_escaflowne_2k · Post by **the_escaflowne_2k** » Mon May 02, 2005 7:03 pm

When it comes to celestial bodies, all light aside from our own solar system is redder than when it is first produced, this is because of the doppler effect. The doppler effect is caused by the "stretching" of light waves, this stretching causes an increase in the wavelength of the light causing it to appear marginally redder. The stretching effect only occurs when objects are moving away from us, thus if all bodies in the univerese are "red shifted" than all bodies in the universe are moving away from us, and thus the universe is in a state of expansion.

The reason light from stars are different colours (well red and white) is because of the temperature of the respective stars, red giants are red becuase they are cooler than normal stars , the hotter an object is the shorter the wavelengths of light emited are. Its the same principal as heating a metal bar, first it only emits wavelengths at a length that are invisible to the naked eye and only detectable with IR devices, then as it heats it glows red (the shortest visible wavelength), then as it gets hotter it emits a yellow white light, then if it could be prevented from vaporising it would glow a blue white and then pure white.

everything that is hotter than -272.15 celcius emits radiation of sorts but untill it gets hot enough that radiation is undetectable.

Finally a degree in Chemistry comes in usefull.

Reflector · Post by **Reflector** » Tue May 03, 2005 12:55 am

Damn, UW. That first post is painful.

Pardon me if I misscience. This is what I recall from Physics II...

Originally posted by The HeartBrend Kid
Light can exist in several different colours, iirc. UV, Infra red, etc...

I wouldn't call ultraviolet and infrared colors of light; I've been taught to think of them as cousins or somesuch. Visible light is electromagnetic radiation in the wavelength range of approximately 400-700 nanometers. Infrared is past the bottom of that range, and ultraviolet is past the top. X-rays, microwaves, gamma rays, cosmic rays, and a whole bunch of other stuff also fall under the heading of EMR. They simply have different frequencies/wavelengths.

Originally posted by the_escaflowne_2k
When it comes to celestial bodies, all light aside from our own solar system is redder than when it is first produced, this is because of the doppler effect.

That is easily the most interesting thing I've read in weeks. Thanks.

Post by **Hound** » Tue May 03, 2005 1:42 am

Ok so stars don't appear red for the same reason an apple appears red?

It's my understanding that say... an apple appears red because it's surface absorbs all other wavelengths of color except red, which is the wavelength it reflects back at your eye.

But a star isn't reflecting light from any other source. So it only gives off the wavelength of visible light that is red? Or am I completely misunderstanding?

ThePeacemkr2 · Post by **ThePeacemkr2** » Tue May 03, 2005 1:56 am

Originally posted by Hound
Ok so stars don't appear red for the same reason an apple appears red?

It's my understanding that say... an apple appears red because it's surface absorbs all other wavelengths of color except red, which is the wavelength it reflects back at your eye.

But a star isn't reflecting light from any other source. So it only gives off the wavelength of visible light that is red? Or am I completely misunderstanding?

It's the doppler effect. Basically, the colors you see are based on the "wavelength" of the light you are seeing. On one end of the spectrum is red, the other, blue.

Stars are moving very quickly away from you. This means that the light they produce has a longer wavelength, by the time it reaches you (sort of like if you fire a bullet from a fast vehicle backwards, it will travel faster than if you fire it forwards, theoretically). They appear red because of that - the wavelength is lengthened in relation to you.

If the star was moving closer to you, on the other hand, the light it produces would have a blue tint, because the light wavelengths would be shorter.

This is from memory, but I think it's somewhat accurate.

Post by **Clay** » Tue May 03, 2005 2:20 am

Originally posted by Hound
Seriously...
What is light reflecting off of or through that gives it that color?

It's reflecting off of the elements within the star itself. When you look at a light surrouded by a gaseous element through a spectroscope, you can actually tell what the element is from what it does to the spectrum. You can also tell how hot the star is.

Bright lines=hot, dense gases
Dark lines=hot, not dense gases
Emission=cold, not dense gases

(I believe; going from memory there).

More than that, light produces a doppler effect, so you can whether or not a star is moving toward or away from you by the color tint. This is called red-shifting.

Oh, and just as an aside:

Light is both a particle and a wave at the same time. Food for thought.

Sir Auros · Post by **Sir Auros** » Tue May 03, 2005 2:24 am

Last time I thought about the nature of light, I realized that everything is essentially dark and I got really depressed. F*ck me if I can figure out how I came to that conclusion now though.

If I had been the first to answer this thread, I would've said "fairy magic."

Post by **Hound** » Tue May 03, 2005 2:28 am

Yes, that I get.

I've got a cool set of astronomy CDs that explain all about light being a wave and a particle and blue and red shifting amongst other stuff.

I thought there might be something about light reflecting off gases that might cause the color of a star but aren't stars made entirely of helium and hydrogen? Or is it just hydrogen? I mean, isn't there not a great variety of gases found in a star with which to create the variety of colors that stars appear in?

So many questions...

Post by **Clay** » Tue May 03, 2005 2:33 am

Originally posted by Hound
but aren't stars made entirely of helium and hydrogen? Or is it just hydrogen? I mean, isn't there not a great variety of gases found in a star with which to create the variety of colors that stars appear in?

It depends. Second generation stars, like our sun, have heavier elements in them that were created when older stars died. Not a lot, but definately more than just hydrogen and helium.

Ultimate Weapon · Post by **Ultimate Weapon** » Tue May 03, 2005 2:46 am

Originally posted by Reflector
Damn, UW. That first post is painful.

Without darkness there can be no light. Absence of light can be merely an object blocking light. Wereas gravity can destroy it.

Cliffjumper · Post by **Cliffjumper** » Tue May 03, 2005 4:47 am

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the_escaflowne_2k · Post by **the_escaflowne_2k** » Tue May 03, 2005 9:53 am

The colour of most objects stems from what wavelengths they either refelect and absorb. Stars colours come from their heat.

Stars "burn" in the following way, isotopes of hydrogen called deuterium and tritium are heated (via intermolecular collisions caused by the immense pressure they ar eunder), after they reach a certain pressure and temperature the atoms fuse togther (nuclear fusion), by fusing togther they create helium and release a neutron and vast amounts of energy. After eons of carrying on like this the hydrogen begins to run out and the helium starts fusing more often (it allready has been some what prior to this but at no substantial rate), however the helium fusion doesnt release as much energy as the fusion of the hydrogen atoms , thus the star cools and becomes a red giant, also the heavier elements are created during this time like carbons and the heavier metals.

Bouncelot · Post by **Bouncelot** » Tue May 03, 2005 10:02 am

Originally posted by Reflector
I wouldn't call ultraviolet and infrared colors of light; I've been taught to think of them as cousins or somesuch. Visible light is electromagnetic radiation in the wavelength range of approximately 400-700 nanometers. Infrared is past the bottom of that range, and ultraviolet is past the top. X-rays, microwaves, gamma rays, cosmic rays, and a whole bunch of other stuff also fall under the heading of EMR. They simply have different frequencies/wavelengths.

They're all part of the electromagnetic spectrum, and though electromagnetic waves/particles have slightly different properties depending on wavelength, Infra Red and Ultra Violet are similar enough that they can reasonably be claimed to be different wavelengths of light. There ar e plenty of animals who can see things that are well into one or both portions of the spectrum.

As for light being both a wave and a particle, so is matter - the atoms that make up your body exhibit wave behaviour as well as particle behaviour.

Axe · Post by **Axe** » Wed May 04, 2005 2:54 am

Wow, this is a nice Black Body Radiation applet:

http://webphysics.davidson.edu/alumni/M ... bb_mjl.htm

All objects emit EMR since their particles are in motion, even "dang cold" uns, say -100 C because thats still 173 K. I remember I calculated air molecules speed to be about 3 times that of an airliner plane at room temperate (a long time ago, Im not sure if I was careful with the calculation). The relationship of temperature and speed is:

square of speed = constant * Temperature in Kelvins

So if you take 300 K (27 C) and reduce to 75, then you'll get half the speed (1.5 times that of an airliner plane).

Black Body Radiation refers to the maximum "black" configuration (increased blackness -> increased EMR absorption and emittion). I remember there was an exercise in a textbook that held that its not a bad approximation to take the sun as a black body (with regard to the overall patern only, we know very narrow frequncy bands are observed, corresponding to electronic energy levels of Hydrogen). The exercise said the sun (surface) was around 5000 K. I remember the bulb filament is around 2500, while im guessing toaster bars would be around 750 K.

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