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I hope this isn't confusing and I'll try to make it as understandable as I can. I would also like to say from the outset, that I'm using a 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
Hello Frank,

If the power output of the laser is 10 watts/ft^2, any location on the metal sheet that is being hit with the laser's beam is getting 10 watts/ft^2. That is true for as long as the beam is hitting that location. If the beam is small, the beam will have to move rapidly if the entire 100 ft^2 of the sheet is to be illuminated before 1 second expires. But that does not mean that every location was briefly illuminated with the laser's 10 watts/ft^2 intensity. So I would say that the peak pulse of power per square foot on the metal sheet is 10 watts.

The fact that the illumination of any location is for a brief period means that the energy received at that location was small, but the intensity, while it lasted, was 10 watts/ft^2.

I hope this helps,
Steve

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Steve Johnson

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I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

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I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

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BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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