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Physics/peak pulse question


QUESTION: Hi, I hope this isn't confusing and I'll try to make it as understandable as possible. Also I'd like to say from the outset, that I'm using a purely hypothetical situation to help get my point across so that you'll better understand what I'm asking. Suppose you have a laser which emits 10 watts per square foot of photon energy. (please remember again that this is hypothetical) The laser is aimed at a metal sheet that is 100 square feet. Suppose the laser beams light to this sheet in such a way, that the laser light is able to hit all of the metal sheet in 1 second. (think of how the electron gun in a cathode ray TV scans the entire screen back and forth rapidly, same idea) Now, because of inverse square law, 10 watts per square foot from the laser scanning 100 square feet in one second, then each square foot of the metal sheet should have gotten hit with the equivalent of  0.1 watts. Now here is my question. Since each square foot of the metal sheet got 0.1 watts of photon power, but over a period of 100th of a second (100 square feet/ 1 second = .001 seconds per sq. ft), wouldn't there be a peak pulse of 1 watt per square foot on the metal sheet?

ANSWER: Not really.  Say you had a 1 sq ft laser profile and you used a beam scanning device (rotating mirrors, like the thing in the supermarket scanners) to scan across your 10 ft x 10 ft (100 sq ft) surface in 10 rows (1 ft wide).  Each sq ft of surface gets the laser for 0.01 seconds, as you said.  During that time, the power is 10 watts, from the laser.  It lasts 1/100th of a second, giving an average power over the whole second of 0.1 watts.  Where you say 100 sq ft/1 second...when you invert that you get 0.01 seconds per sq ft and not 0.001 seconds per sq ft.  Multiply by your 10 W/sq ft and you still end up with 0.1 W for average power.  Nothing hinky.

---------- FOLLOW-UP ----------

QUESTION: But would there be a peak pulse in addition to the average?

ANSWER: The "peak pulse" would be 10 W/sq ft, when the laser was on that spot.  You never mentioned varying the laser power, so it would be that or zero intensity during the 1 second time frame.

---------- FOLLOW-UP ----------

QUESTION: Zero intensity? Could you explain what you mean by that? Thank you!

As you specified, the laser only shines on one square foot of the 100 sq ft at a time.  Therefore, that square foot is receiving the specified 10 W/sq ft and the rest is receiving zero as your laser scans over the surface.  It can only point one place at a time.


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Dr. Stephen O. Nelson


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.


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.

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

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