Newbie questions on laser

Dear all, hope you can help me resolve my doubts.

1) If a laser can light up a paper in 10m, can the same laser light up the paper in 100m? Does distance play a role here?
2) Is a 100mw red laser as powerful as a 100mw green laser in terms of burning ability?
3) Is there any reliable seller in taobao/china which sell strong laser at low price?

lipo99 said:

Dear all, hope you can help me resolve my doubts.

1) If a laser can light up a paper in 10m, can the same laser light up the paper in 100m? Does distance play a role here?
2) Is a 100mw red laser as powerful as a 100mw green laser in terms of burning ability?
3) Is there any reliable seller in taobao/china which sell strong laser at low price?

Click to expand...

1- all depends on the power of the laser. for hobby/home use, i don't think there is a laser that can burn at 100m. For military, yes.
2- yes and no. depends on what color the object is
3- use google.

Laser beam diameter increases with distance, basically in linear way. It's called divergence. Common pointers have divergence about 1mRad (milliradian), which roughly means that for any distance the dot size will be 1/1000 of the distance .. ie. at 100m it will be 0.1m in diameter. All laser power is distributed over the spot area .. and spot area grows in quadratic way with increasing diameter. So if you double distance, spot size will double, but power density will decrease 4 times.
That is without considering dispersion of light in the air, which affects more short wavelengths. It should not be big problem on 100m though.
Laser beam divergence increases with wavelength - shorter wavelength is better .. and decreases with laser aperture diameter - large aperture means smaller spot (it's the same reason why astronomic telescopes are large to have good resolution .. just the light is moving in opposite direction).