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I shined a green laser pointer at an orange ping pong ball today and saw a distinctly bright yellow color.

At first I thought about it and reasoned that green=(yellow + blue) hits the ball which reflects orange=(yellow + red) thus yellow is reflected.

Then I thought, wouldn't this mean that a prism would break a green laser into yellow and blue? A quick search confirmed this is obviously not the case since laser light is a single wavelength, not an additive combination of primaries...

What then is a good explanation for this? enter image description here

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    $\begingroup$ This is definitely not going to be as easy as green=yellow+blue, because atoms don't share our color perception when they figure out their energy levels. This physics.stackexchange.com/q/72560 is a very similar question you probably want to look at. $\endgroup$ – bob.sacamento Sep 12 '17 at 21:17
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Laser light from a green laser pointer is essentially monochromatic. I demonstrated that in this earlier answer which shows how to measure the wavelength - in the process of that experiment you see the spectrum consists of a single peak.

I hypothesize that your orange pingpong ball is fluorescent (this is often done to make the ball more visible: see this article for more details. As that article mentions, white balls are used in tournaments, where the table, floor and clothing are dark-colored, lighting is sufficient, and white will provide better contrast).

As the green light has a shorter wavelength (higher energy) than the emission wavelength of the phosphor, you excite the phosphor in the ball - and it then emits the longer wavelength of light.

You see a similar thing if you use a 405 nm (almost invisible, near UV) laser pointer and shine it at a piece of white paper. The paper contains compounds that fluoresce, and the resulting beam suddenly goes from "barely visible purple" to "bright blue".

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  • $\begingroup$ High quality business cards make great UV laser viewing cards (until you burn a hole in them). $\endgroup$ – Jon Custer Sep 12 '17 at 21:22
  • $\begingroup$ @JonCuster I didn't know that. Watch your eyes - a UV laser nearly cost me my eye sight. $\endgroup$ – Floris Sep 12 '17 at 21:22
  • $\begingroup$ You beat me to it. It will be fluorescence, possibly RFP which is excited by 532nm lasers. The reason for this will be to make the ball optically brighter under light which has little or no UV but a decent amount of green: indoors, in other words. $\endgroup$ – tfb Sep 12 '17 at 21:25
  • $\begingroup$ @tfb you are absolutely right - it is intended to make the ball more visible. I will add that to my answer. In my native language, we would call such a color "lichtgevend" (giving off light) because the object would often look unnaturally bright. Especially if it could convert some invisible (UV) light from the environment. $\endgroup$ – Floris Sep 12 '17 at 22:04
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    $\begingroup$ Absolutely! As the (grad student) laser safety officer with a 4J/pulse YAG laser with double/triple/quadrupled output wavelengths we were very respectful and careful... $\endgroup$ – Jon Custer Sep 12 '17 at 23:48

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