# Night Sky

**URL:** <https://boards.straightdope.com/t/night-sky/341>\
**Category:** Cecil's Columns/Staff Reports\
**Created:** [December 8, 1999, 4:34pm UTC](https://boards.straightdope.com/t/night-sky/341 "1999-12-08T16:34:00Z")\
**Posts on this page:** 1\
**Page:** 2

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**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\
**Post date:** [December 14, 1999, 3:55pm UTC](https://boards.straightdope.com/t/night-sky/341/21 "1999-12-14T15:55:00Z")

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TheDude

> [@](#):
>
> Not really parallel, but all pointing in the same general direction. However, the light from stars travels far enough before it gets here that the light rays are essentially coherent - all aligned in parallel rays. This is process that is emulated to make laser coherent. They use tubes with mirrors to simulate a long travel distance to let only the coherent light out.

> [@](#):
>
> Just nitpicking here, but this is not exactly true. While it is true that the laser does use a tube with mirrors to only let coherent light out, the important part of a laser is embedded in the etymology of the word-Light Amplification by Stimulated Emission of Radiation. That is, the laser tube doesn’t just act as a orientation filter, but it also acts to amplify only the light that is oriented along the axis of the tube.
> 
> If one were simply to place a light source in a tube with mirrors on the ends, it would still act as a point source of light and not as a source of coherent light.

Righteo - I didn’t mean to imply that was the only significant part of laser generation.

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