Thread: Another question about light

in principle, distance doesn't affect doppler shifting. It's the speed difference of the emmitter and the reciever. I imagine that if you saw red and green airplane lights, then they really were red and green

As far as the question about firefly lights goes, I'm not sure how to explain that. How many were there? how many times did you see them?

My nephew and I saw white lights in the sky about two days ago. They blinked like small sparks from fireworks and sat still in the sky. Sometimes they blinked out and reappeared a couple of meters away. And other times they were so bright they looked like small lightning strikes.

I'm guessing there was more than one doing this (to make it seem like it jumped). But I only saw one appear at a time, and the others did not reappear. It traveled in a path by "Blinking out".

When I first saw it, I thought it was like a comet, but it had a thick trail like someome took a marker and slashed the sky slowly. It also turned around and went into the trees.

There was fireflies that night, and that's the only explanation I can come up with.

In the dark, it'd be hard to see the fireflies' bodies. So, yes, you'd only see the little white light. You can also sometimes see fireflies from a surprising distance away. They don't flash very often, which could explain why they didn't happen to flash at the same time as another one.

haven't you ever seen a red star? Light doesn't lose its color at a distance.

From where I live there are few stars. Can't say that I seen a red star, plenty of white ones though.

Originally Posted by Xaqaria

haven't you ever seen a red star? Light doesn't lose its color at a distance.

That's not true. Light doppler shifts. If the source of the light is approaching you at speeds that are not neglibable compared to the speed of light, it will shift in a blue direction. If it's moving away from you at non trivial speeds, it will shift in the red direction. This is one of the ways that we prove that the universe is expanding.

While we're talking about light, I've been thinking about something:

If there are 3 colors of light (red, blue, and green), and a thing is the color of the light it reflects, what if there is no light of that color?

Say you have a completely red cube. It reflects red light and nothing else at all. If you took it into a dark room and lit a blue light, would it be invisible, or would it be black?

Originally Posted by Souperman22

While we're talking about light, I've been thinking about something:

If there are 3 colors of light (red, blue, and green), and a thing is the color of the light it reflects, what if there is no light of that color?

Say you have a completely red cube. It reflects red light and nothing else at all. If you took it into a dark room and lit a blue light, would it be invisible, or would it be black?

It would be "black". If there's nothing behind it that reflects the blue light to let you see the silhouette, then it will be "invisible", in that you'll see nothing but blackness.

Oh, and there are not just three colors of light, it is a continuum. The whole Red Green Blue thing is a human vision attribute, we differentiate light into only three slightly overlapped bands of frequencies, rather than processing light frequencies in the way that our ears process sound frequencies. That's a good thing too in a way, if our eyes processed light frequencies as well as our ears do sound, we still wouldn't have full color monitors, and monochrome would look orders of magnitude more bland than it does to us with our three band frequency perception.

I disagree but not definitively. The red object is absorbing the blue light that it doesn't reflect. The atoms do this and it moves the electrons in the atoms into a 'higher' orbit. They will decay into a lower orbit and conservation of energy demands that they release a photon to do so. The frequency of this photon will depend on the atom that is in question. Given atoms only have particular orbits open for an electron to occupy and moving between them 'costs' a discrete amount of energy that gets tranformed into the photon. The amount of energy determines the color of the photon.

The above is definitive as per quantum physics as we currently understand it. The amount of photons being released may not be enough to be visible but they will be detectable with equipment. I think the question of if it would be visible or not depends on the elements involved. Unfortunately, I still haven't studied quantum physics with enough rigour to be able to make the calculations for a given marterial

So we don't have a definitive answer.

Originally Posted by PhilosopherStoned

That's not true. Light doppler shifts. If the source of the light is approaching you at speeds that are not neglibable compared to the speed of light, it will shift in a blue direction. If it's moving away from you at non trivial speeds, it will shift in the red direction. This is one of the ways that we prove that the universe is expanding.

The doppler shift has nothing to do with light 'losing color' at a distance. No matter how far away a light source is, the shift in frequencies will only be dependent on how fast the source is moving away from/towards the observer.

Originally Posted by PhilosopherStoned

I disagree but not definitively. The red object is absorbing the blue light that it doesn't reflect. The atoms do this and it moves the electrons in the atoms into a 'higher' orbit. They will decay into a lower orbit and conservation of energy demands that they release a photon to do so. The frequency of this photon will depend on the atom that is in question. Given atoms only have particular orbits open for an electron to occupy and moving between them 'costs' a discrete amount of energy that gets tranformed into the photon. The amount of energy determines the color of the photon.

The above is definitive as per quantum physics as we currently understand it. The amount of photons being released may not be enough to be visible but they will be detectable with equipment. I think the question of if it would be visible or not depends on the elements involved. Unfortunately, I still haven't studied quantum physics with enough rigour to be able to make the calculations for a given marterial

So we don't have a definitive answer.

If an object is red, and it absorbs blue light, it will emit a photon outside the visible spectrum (infrared heat). It will still appear black.

Originally Posted by Xaqaria

The doppler shift has nothing to do with light 'losing color' at a distance. No matter how far away a light source is, the shift in frequencies will only be dependent on how fast the source is moving away from/towards the observer.

that's what I said and that's doppler shifting.

EDIT: Never mind. I see what you were saying in the quote that I was responding too. I was putting words in your mouth. As a heuristic, the further away a star is, the faster it is moving away from us.

If an object is red, and it absorbs blue light, it will emit a photon outside the visible spectrum (infrared heat). It will still appear black.

cool.