Unusual Color Effect of Laser Pointer on Red Plastic: A Scientific Investigation

[Mu naught]

so I was playing with my 532 nm (green) laser pointer when I noticed something strange. You can shine it at virtually any surface it will appear as a bright green dot - nothing unexpected there. However by chance, I noticed that when I pointed it at this red bottle of laundry detergent i have, instead of appearing as a green dot, it changes very noticeably to an orange/yellow color. Yet when I shine it at an oblique angle so I can see the reflected light on another surface, the reflected light is green.

Here is a video I took of it. It's not the best quality and you'll have to believe me when I say that the color change is VERY noticeable by eye.



My only explanation is that something in the plastic is absorbing the green light and then emitting light of a different wavelength. When the new color mixes with the reflected green light it forms the yellow/orange color.

If you have a better explanation please post it.

 

[NobodySpecial]

I don't think so - it's more an intensity effect.
The green laser light should in theory only be detected by the green sensor in your camera, and when you shine it on red you should see nothing. The red bottle only reflects red light, the green laser contains no red.

But the red bottle will reflect a little green and the red sensor in your camera will detect a little green - so when the amount of green is reduced the other colors are visible.

 

[Mu naught]

No. I have shined it at other red surfaces and it appears green. According to your theory, every non-white surface would look different.

Plus it has nothing to do with the camera, I told you it's much more pronounced by eye.

 

[NobodySpecial]

Then it is probably a fluorescence effect in the colored dye in the plastic.
The green light is exciting electrons in the dye molecules which then fall back giving out redder light. You can only get a more red color because red photons have less energy than green/blue ones and the process isn't perfectly efficient - some energy is lost.

 

[Cleonis]

so I was playing with my 532 nm (green) laser pointer [...] when I pointed it at this red bottle [...] it changes very noticeably to an orange/yellow color. Yet when I shine it at an oblique angle so I can see the reflected light on another surface, the reflected light is green.
[...]
My only explanation is that something in the plastic is absorbing the green light and then emitting light of a different wavelength.

I think so too; the explanation that you offer seems to be the only candidate.

By the way, the part of the light that is reflected does not penetrate into the bottle's material, hence its composition remains unaltered. In that sense it's not surprising (indeed expected) that reflected light remains unaltered.

Any chance of using a prism to confirm the altered composition of the light?

 

[Mu naught]

I think so too; the explanation that you offer seems to be the only candidate.

By the way, the part of the light that is reflected does not penetrate into the bottle's material, hence its composition remains unaltered. In that sense it's not surprising (indeed expected) that reflected light remains unaltered.

Any chance of using a prism to confirm the altered composition of the light?

I thought of that but I don't have one. I can probably find one lying around campus and post an update.

Then it is probably a fluorescence effect in the colored dye in the plastic.
The green light is exciting electrons in the dye molecules which then fall back giving out redder light. You can only get a more red color because red photons have less energy than green/blue ones and the process isn't perfectly efficient - some energy is lost.

So the lost energy goes where? Into heat?

 

[Mu naught]

Update: I realized I could just use a CD to produce a spectrum. I shined the laser at the bottle again, and then I also used a red notebook as a control.

Here is the result:

Bottle:

Notebook:

So yup it looks like the bottle is emitting yellow/red light when the green light is shined on it. There is a little bit of red light from the notebook too - I think that may just be reflected sunlight because it's not perfectly dark in my room.

I guess this thread is pointless now though but this was fun for me!

 

[NobodySpecial]

So the lost energy goes where? Into heat?

Basically yes.

It is actually possible to end up with a bluer photon by the way. There are some materials where if you hit the electron with two lower energy photons quickly enough it can then emit an energy equal to the sum of them. This is how some blue/green lasers work - they have a high power red/infrared laser inside and a piece of this material which gives out a photon of half the wavelength - although the efficency is low and you waste a lot of power.

 

[hl_world]

Deep green light (especially from green lasers) always illuminates fluorescent surfaces with a red enough hue. It's widely mistaken that only UV light does this.

There are some materials where if you hit the electron with two lower energy photons quickly enough it can then emit an energy equal to the sum of them. This is how some blue/green lasers work - they have a high power red/infrared laser inside and a piece of this material which gives out a photon of half the wavelength - although the efficency is low and you waste a lot of power.

Although some infrared light is still emitted with the green light if there is no IR filter which, apparently, is even more harmful to the eyes than the main green. You might see it as a dim red colour when the batteries run dry. It's only dim because the cells on your retina that actually sees light aren't sensitive to it but it can still damage other retinal cells easily and make you blind so don't be tempted to look into the dim red beam.

 

[ChrisAndre]

This also seems to work with digital projectors; the light reflected off the screen and then reflected off the floor appears only green.