Quick Physics Question

[Urban Ranger]

Originally posted by Boris Godunov
I suppose that, if you eliminated all possible impediments and had an absolutely perfect and straight laser beam, the limit of seeing it would be based on the distance of the viewer from the laser source and the speed of light.

And intensity of the source.

[JellyBean]

Even perfectly coherent light will suffer from diffraction. When the distance L between the light source and the observer is much greater (100 times or so) than the original width of the beam D and the wavelength W of the light, there will be a central spot of light illuminating the observer with a fainter series of concentric rings around it. The central spot will have a radius of W*L/D.

For example, if the torch is a laser shining a beam of green light (wavelength of 500nm or 5*10-7m) with a diameter of 5cm (5*10^-2m), at a distance of 10km (10^4m) most of the light will be spread out over a circle with a radius of 10^-1m or 10cm.

See http://www.colorado.edu/physics/phys...html/lab5.html for a good explanation of this - scroll down until you get to the bit about a single-slit diffraction pattern.

Edit: Learned how to put in a link on these boards.

[MichaeltheGreat]

Originally posted by Jack the Bodiless
...Ugh. Insufficient data. How bright is the flashlight, how tight is the beam, how good are the observer's eyes?

If it's pointed at you, in undisturbed visibility conditions, a typical three D cell maglite can be seen on a dark night from 10 km distance on earth by someone with normal vision. Army issue flashlights with white light can be seen at a little over half that range.

Jelly Bean is absolutely correct, also.

[Vince278]

Re: Quick Physics Question

Originally posted by Eli
If I hover above the moon at a distance of, say, 10km from the surfrace and turn on my flashlight and point it downwards, will someone who's standing on the moon directly below me be able to clearly see the light because the moon has no atmosphere?

If the answer is yes, how far will I have to be for him to not see the light(assuming nothing blocks it except couple straying hydrogen atoms), and why wont he see it?

Will the light appear weaker at a distance? Yes
Will the light have the same appearent magnitude? Yes