You are here:

# Physics/Photons and lasers

Question
Sir,

A street vendor showed me a green pen laser, to show me how powerful it was, he directed the laser at a an object...very far away. I can clearly see the green spot. I am wondering..what if I pointed the laser at the moon..obviously a thought experiment. I assume that the energy would be dissipated on the journey and there would not be a green spot on the moon. My question is..would ANY Photon from the pen..at all, ever reach the moon?

Tq.

So light scattered off of the surface of an object has an intensity that is scaled by the square of the cosine of the scattering angle.  If that doesn't make sense, don't worry...it basically means that it's most intense if it's scattered back at you.  But will any photon reach Earth from a laser pointer?  Yes, over time, but not enough to see.

My friends and I had this idea a long time ago, in a contest to figure out the coolest marriage proposal...I said "write it on the clouds over NYC!"  My friend Neil one-upped me with "what about the new moon?  Forgetting that the new moon is only up during daylight hours, pretty much, we started the power calculations (being good physics students).  Turns out that because the moon is 384,000 km away (average), it would require gigawatts of laser power at that scattering profile to see here on Earth.  Continuous gigawatts, not just pulsed.  So while technically you could shine your laser on the moon and have it reflected to Earth...you won't see it with the naked eye at all.  That thing they did on the Big Bang Theory where they bounced a laser off of reflectors on the moon is for real, but requires a sensitive photomultiplier tube to pull off as a receiver.

Physics

Volunteer

#### Dr. Stephen O. Nelson

##### Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

##### Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Education/Credentials
Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.