Question
Tue February 21, 2012 By:
 

an astronaut flashed a laser light and a tube light from space towards the earth. the laser light appear to twinkle but the tube light didn't. why????

Expert Reply
Wed February 22, 2012
It takes time for our brains to understand what our eyes see.  Because of that, a fast moving laser beam spot will not look like a spot at all, but like a line of light.  If a laser beam is moved fast enough, the beam spot on a wall can be made to look like a circle or some other figure.  However, if you took a photograph of it, and if the camera is fast enough, it will clearly show just a spot.  Movies and TV are just a series of still pictures flashed quickly on the screen.  The flashing is fast enough so that it looks like smooth motion to us.
 Mostly we see light from the sun or other sources that bounces off of objects.  Occasionally, objects emit their own light.  Light travels in straight lines.  A laser produces light, but unless the laser light bounces off of something, or shines directly into your eyes (a very bad idea, since this can damage your eyes), you cannot see it.  The beam is invisible in normal air.  However, you can see the beam if there is something in the air, like smoke, that the light can bounce off of.  
As laser have shorter wavelenghts than the visible range, they tend to lose energy faster than the visible light, they tend to change the direction as they enter differen tmediums in the atmosphere, so they look like they twinkle.
One useful informatin on laser:
Stars twinkle because air currents driven by the evaporation of water cause their light to shimmer as it passes through the Earth’s atmosphere. (To avoid this twinkling, large observatories are sited on remote, dry mountaintops in places like Chile.) On a smaller scale, the light from Dr Kleissl’s laser also shimmers. The more water evaporating from a field, the more the laser’s light appears to twinkle. This, when properly interpreted, can tell the field’s owner how damp the soil is and thus whether he needs to switch the sprinkler on, and for how long.
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