Last night I was looking up at the stars with my stepdaughter and we were talking about the constellations, noticing that some stars were hard for us to see while others were very bright. Then this question occurred to me. If light travels, why should the light from distant stars be dimmer than light from closer stars? A car that came from far away doesn't arrive smaller than a car that came from close by! Could anyone, er, shed some light on this for me?

The apparent brightness of a star depends on a number of factors.....it's size, temperature, distance from Earth, etc. If you take a 1000 watt light bulb and place it say 10 feet away from you, it's going to appear pretty bright. Now take that same bulb and have it placed a 1000 feet away from you, and it will appear very dim, or perhaps not able to even be seen. That is because it's light (radiation energy) dissipates over distance, that is, it spreads out over a larger area, and less energy reaches your eyes. Same with a flashlight..it works good at close range, but it's useless when searching for something 100 feet away. And when you think for example that the guiding light of the North Star travels over 2000 trillion miles to reach us, it's a wonder we can see it at all!
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04-12-05, 11:49 AM
Kendor

quote:
Originally posted by gerry:
That is because it's light (radiation energy) dissipates over distance, that is, it spreads out over a larger area, and less energy reaches your eyes.



With this in mind Sarai, now think about laser light. Laser light is concentrated in one direction, much like your car analogy on your OP. It maintains its brightness over long distances.

04-12-05, 12:36 PM
Sarai
Interesting! Does that mean that theoretically, if someone shines a laser into outer space, that light will shine on out into the universe infinitely (or until the end of the universe, whichever is the case)?

04-12-05, 03:55 PM
gerry
I don't know about lasers, but the Hubble Telescope has observed light from galaxies in their early formative stages oh about a billion years after the Universe was created, light that is just now reaching its lens after having travelled some 13 billion years and 80 billion trillion miles to get here. Now that's quite a distance, I would say !

04-12-05, 04:24 PM
Sarai
Hmm. Food for thought. Thanks, Gerry and Kendor. Smile

04-12-05, 08:19 PM
methos
In response to the laser question - even a laser is not perfectly collimated and therefore broadens. I remember doing an experiment in college measuring the spread of a laser at different distances and for a good laser it is a very slow broadening.

However, that's relative. They've bounced lasers off of mirrors on the moon and those beams are over a mile wide by the time they make it that far (and nearly 10 miles wide by the time they reach the Earth again).

Then there is loss due to scattering. If you can see a laser beam but it is not pointed directly at your eye (which it had better not be as this is obviously dangerous), you are seeing the fraction of the beam that has scattered off of particles in the atmosphere.



Now, as to why the light gets dimmer, this was correctly answered but I thought I might expand on it.

First way of thinking about it:
Imagine (or even draw) straight lines of light coming from a star and going in all directions. Each line can represent the path of a photon, or a single particle of light. The more photons hit your eye, the brighter the star seems. If you can imagine (or look at the drawing), the lines are closer together near the star than they are further away from the star, so it seems brighter close up.

Second way of thinking about it:
Assuming the light isn't removed somehow (absorbed or scattered by some object blocking your view) it'll continue forever. Enclose the star at the center of a ball 1 lightyear wide (no need to try to conceptualize exactly how big that is, it's not important). The light coming from the star will be spread roughly evenly across the entire inner surface area of the ball. Now replace that ball with a much larger one... 10 light-years wide. The total light reaching the surface will be the exact same, but the surface is much much larger (100 times larger), so it is much more spread out. Now imagine that your eye is a small area on the surface of either of these spheres. The light will be 100 times less bright when your eye is where the large ball was than it would be when your eye was where the smaller balls was.

Adding to both of these is that the light doesn't go on undisturbed forever. We think of space as empty, but when you consider how much space there is between us and even the closest stars, there's plenty of stuff to scatter or absorb a the light, making it dimmer and dimmer the further away it is (just like the laser). And, of course, there's quite a bit of atmosphere to scatter the light if you're watching from Earth.

04-12-05, 08:38 PM
methos
Not directly important to the question, but I should clarify something. When I say brightness I don't mean as your eye perceives it but rather absolute intensity (quantity of light).

Your eye actually perceives light an a scale more like the way we record earthquakes (Richter) or sounds (decibel) so that something that seems twice as bright to us in reality is many times brighter.

04-12-05, 09:37 PM
gerry
Speaking of lasers, why is it that some can burn holes through steel, while others are blinding but not hot at all, and still others, like the one my dentist uses on a root canal (?)which is bluish in color, is neither bright nor hot? And what about the eye surgery lasers??

04-12-05, 10:59 PM
methos
It's a matter of both power and wavelength. Most lasers we come into contact with are class 1 lasers like laser pointers, which are too weak to do damage (though I would still be cautous with them).

Obviously, higher powers can do more damage, but wavelength complicates this. First, green lasers will appear to be brighter than blue or red lasers of the same power becasue your eye is more sensitive to green. Of course, ultraviolet and infrared lasers are invisible to our eyes.

Different wavelengths can cause different effects. For example, in your eye:
Visible (especially green) and near-infrared light will damage your retina. Your retina is especially vulnerable to this light because the cornea and lens will not absorb it but will focus it onto your retina, effectively increasing the density of power by 100,000 times or so. Other infrared light and mid-ultraviolet will damage your cornea, but won't necessarily make it to the retina because the cornea absorbs it. Near-ultraviolet will damage the lens, but also won't necessarily make it to the retina.

04-13-05, 04:24 PM
frankvan
One tiny bit of information that hasn't been mentioned is that because the light that reaches us from outer space has to travel through an atmosphere that is fluid and constantly moving, through convection currents on even still nights. That light is refracted and bent just like the light coming from an object ahead of the viewer passing over a hot pavement appears to shimmer or undulate. A similar effect causes the stars to appear to twinkle.

I remember a college experiment in which a black ballon inflated inside of an inflated clear balloon, hit with a red laser beam caused the black baloon to burst while the outer, clear baloon remained intact. The same reason a black object gets red hot in the sunlight while an adjacent white object remains quite cool.

04-13-05, 05:02 PM
Sarai
This is so interesting! So... will someone please invent a time machine so that I can go back and study physics properly? Thanks! Wink

04-13-05, 08:48 PM
Professor

quote:
Originally posted by Sarai:
A car that came from far away doesn't arrive smaller than a car that came from close by! Smile

No, but its looks smaller (and therefore dimmer) when it's far away, for the reasons explained above. Individual photons (assuming they're the same color) are all identical in "size." Brightness is perceived as how often the photons strike your retinas. The very dimmest objects (invisible to the naked eye) are seen just one photon at a time!

Consider also: When you gaze at a star, each photon has traveled for up to billions of years to complete its long journey by relinquishing its energy just in your retina and nowhere else. The person standing next to you sees the same star by way of different photons.

Ain't it grand? Wink

04-13-05, 11:21 PM
methos
Two more things to consider regarding the why some lasers do more damage to certain things than others.

Although we talk about lasers as a narrow beam of highly collimated (and coherent) light, this is only an ideal laser. The degree to which a real laser approximates this will influence how it affects surfaces (a more tightly focused beam will deposite more energy in a smaller area, for example).

Lasers also may be pulsed or continuous. Two lasers may deposit the same amount of energy into a surface over the course of a second, but one that deposits this evenly in time (a continuous wave laser) would affect a surface differently than one that deposited it in a series of more intense but only picosecond long pulses.

04-18-05, 02:46 PM
Pin~Jinx
Well answered Gerry, Smile

umn, now considering 'light' as an energy,
wouldn't we assume it looses its Kinetic Energy as it travels...

...and learnt as a kid that distance and intensity have an inverse relationship (although maybe that was with relation to the Electric or Magnetic fields) but wouldn't that apply here too???

Pin~Jinx / anarchist

04-19-05, 04:15 PM
gerry
Light loses energy, but not kinetic energy; Light (photons) has no mass, and since kinetic energy is a function of mass and speed, it has none. The energy it loses is mostly due to absorbtion by other particles or objects in its path...usually this loss is in the form of heat, energy must be conserved. You are right about the inverse law, I believe apparent light and electric/magnetic field intensities reduce as the square of the distance from the source, doubling the distance reduces the fields by 4, etc.

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