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Old 06-10-2008, 12:13 AM #1
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Default Why is the front of a beam brighter than the back?

So my main understanding of light over my life was that it bounces off objects and into our eyes, which is why we see an image. *But for some reason, that logic (to me anyway) doesn't explain why a laser beam is more illuminated viewing front on, than it is viewing from behind. *I mean, to my logic, the laser beam should leave the diode, bounce off the particles of air and into our eyes. *But if the back of the particles of air is being struck by light, why is it the opposite side that's most illuminated? *I mean, the laser beam is going past your head, so your eyes aren't being directly struck by the light, what is it about light that makes it so visible viewing from that angle?


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Old 06-10-2008, 12:25 AM #2
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Default Re: Why is the front of a beam brighter than the b

Well, a laser beam in a vaccuum would be invisible. To see it, it needs to be scattered by particles in the air. This can happen in two ways - you can have particles reflect backwards off the path, or they can refract to the side as they travel forward. The forward refraction is much more notable and is the most intense at smaller angles. So at the 90 degree angle, you get very little of either effect - thus the beam looks weakest from the side. Looking down the beam from the front is brightest because of refraction. Down the beam from behind is also quite bright, but the reflection is not as notable.
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Old 06-10-2008, 12:45 AM #3
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Default Re: Why is the front of a beam brighter than the b

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Originally Posted by pseudonomen137
Well, a laser beam in a vaccuum would be invisible. To see it, it needs to be scattered by particles in the air. This can happen in two ways - you can have particles reflect backwards off the path, or they can refract to the side as they travel forward. The forward refraction is much more notable and is the most intense at smaller angles. So at the 90 degree angle, you get very little of either effect - thus the beam looks weakest from the side. Looking down the beam from the front is brightest because of refraction. Down the beam from behind is also quite bright, but the reflection is not as notable.
Ah, so the particles of light are actually chipping off to the side slightly and hitting us directly? That makes sense, thanks. It's something I never concidered, I only ever thought of reflection...
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Old 06-10-2008, 12:54 AM #4
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Default Re: Why is the front of a beam brighter than the b

If you look in the thread "Why is the sky blue" you'll see a discussion of Rayleigh and Mie scattering. The type you're wondering about is Mie scattering. There's a very good discussion in that thread. Look here: http://www.laserpointerforums.com/fo...12608788/12#13
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Old 06-10-2008, 02:47 AM #5
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Default Re: Why is the front of a beam brighter than the b

Quote:
Originally Posted by pseudonomen137
Well, a laser beam in a vaccuum would be invisible. To see it, it needs to be scattered by particles in the air. This can happen in two ways - you can have particles reflect backwards off the path, or they can refract to the side as they travel forward. The forward refraction is much more notable and is the most intense at smaller angles. So at the 90 degree angle, you get very little of either effect - thus the beam looks weakest from the side. Looking down the beam from the front is brightest because of refraction. Down the beam from behind is also quite bright, but the reflection is not as notable.
That's an awesome explanation! i always wondered why that happened, now i know !
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