# The Naked Scientists Forum

### Author Topic: Why does light dispersewith distance and what keeps laser light from dispersing?  (Read 8460 times)

#### Joe L. Ogan

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##### Why does light dispersewith distance and what keeps laser light from dispersing?
« on: 14/10/2010 19:05:37 »
Why does light disperse with distance and what keeps laser light from dispersing?  Thanks for comments.  Joe L. Ogan

#### lightarrow

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##### Why does light dispersewith distance and what keeps laser light from dispersing?
« Reply #1 on: 14/10/2010 19:49:55 »
Why does light disperse with distance and what keeps laser light from dispersing?  Thanks for comments.  Joe L. Ogan
Are you talking of "dispersion" or "divergence"?

#### Joe L. Ogan

• Sr. Member
• Posts: 476
##### Why does light dispersewith distance and what keeps laser light from dispersing?
« Reply #2 on: 14/10/2010 21:22:38 »
Do you have a preference?  Thanks for comments.  Joe L. Ogan

#### maffsolo

• Sr. Member
• Posts: 280
##### Why does light dispersewith distance and what keeps laser light from dispersing?
« Reply #3 on: 15/10/2010 02:09:58 »
Measuring device so far the best we have.
"Laser beams are used because they remain tightly focused for large distances. Nevertheless, there is enough dispersion of the beam that it is about 7 kilometers in diameter when it reaches the Moon and 20 kilometers in diameter when it returns to Earth. Because of this very weak signal, observations are made for several hours at a time. By averaging the signal for this period, the distance to the Moon can be measured to an accuracy of about 3 centimeters (the average distance from the Earth to the Moon is about 385,000 kilometers)."

Columnated Light from a laser beam has an ordered structure of direction. It is like keeping your ducks in a row from the start.
The light waves will eventually disperse away from the center of the diameter of the beam over a long distance, as its energy dissipates.

Lasers
“The photons involved in the emission process all have the same energy and phase so that the laser beam is monochromatic and coherent, allowing it to be brought to a fine focus”

Non columnated light is random light waves, photons with no ordered structure to the energy or phase, permits them to disperse at random and sooner.

« Last Edit: 15/10/2010 03:10:49 by maffsolo »

#### lightarrow

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##### Why does light dispersewith distance and what keeps laser light from dispersing?
« Reply #4 on: 15/10/2010 11:58:46 »
In optics the term "dispersion" means separation of frequencies in a medium, that is, the phenomenon you oserve when white light goes through a glass prism. What Ogan talks seems the fact a laser beam "remains tightly focused at large distances" as you wrote.

#### lightarrow

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##### Why does light dispersewith distance and what keeps laser light from dispersing?
« Reply #5 on: 15/10/2010 12:01:31 »
Do you have a preference?  Thanks for comments.  Joe L. Ogan
Yes, but just because optics books and physicists have (if it were for me, you could call it as you want).

#### maffsolo

• Sr. Member
• Posts: 280
##### Why does light dispersewith distance and what keeps laser light from dispersing?
« Reply #6 on: 15/10/2010 17:51:48 »
In optics the term "dispersion" means separation of frequencies in a medium, that is, the phenomenon you oserve when white light goes through a glass prism. What Ogan talks seems the fact a laser beam "remains tightly focused at large distances" as you wrote.

"In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency,[1] or alternatively when the group velocity depends on the frequency. Media having such a property are termed dispersive media. Dispersion is sometimes called chromatic dispersion to emphasize its wavelength-dependent nature, or group-velocity dispersion (GVD) to emphasize the role of the group velocity"

Joe is not addressing chromatic dispersion
« Last Edit: 15/10/2010 17:55:14 by maffsolo »

#### maffsolo

• Sr. Member
• Posts: 280
##### Why does light dispersewith distance and what keeps laser light from dispersing?
« Reply #7 on: 15/10/2010 18:02:54 »
Do you have a preference?  Thanks for comments.  Joe L. Ogan
Yes, but just because optics books and physicists have (if it were for me, you could call it as you want).

What is your idea on why a laser beam can stay more tightly focused over a long distance compared to a standard beam of light of equal power over a long distance?

#### lightarrow

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##### Why does light dispersewith distance and what keeps laser light from dispersing?
« Reply #8 on: 15/10/2010 18:49:42 »
What is your idea on why a laser beam can stay more tightly focused over a long distance compared to a standard beam of light of equal power over a long distance?
Because of how a laser (generally) works. The laser beam is generated by the sum of a great number of photons which are all aligned in a precise direction, because the others are suppressed, in the sense that only those aligned along the device's axis are summed all together, infact the em radiation bounce back and fort the little ends of the device many times; it's as if the effective lenght of the device were extremely high and only a parallel beam can escape, hope I could explain the concept.
« Last Edit: 15/10/2010 18:57:20 by lightarrow »

#### The Naked Scientists Forum

##### Why does light dispersewith distance and what keeps laser light from dispersing?
« Reply #8 on: 15/10/2010 18:49:42 »