# COMMUNITY : FORUMS : SCIENCE

## Observing single photons (may be future Instructable)

In a thread on building a DIY Geiger counter, I mentioned that it's possible to do an experiment to see single photons directly with your eyes. This is a lab we did when I was an undergraduate, more than 20 years ago. I haven't done the setup myself since then, so I'm just going to describe it; if I have the opportunity run it again, then I'll make an I'ble.

If someone else decides to tackle it, please feel free to write it up yourself!

The human eye detects light via a family of proteins called opsins. Different forms of photopsins are sensitive to different wavebands, which is what gives us color vision. Rhodopsin is sensitive mainly to greenish-blue light, and provides us with monochromatic night vision. Rhodopsin works by changing its conformation when it absorbs a photon; that change of conformation allows ions to flow through the rod cell's membrane and generate a signal. The signal from each rod cell is processed through the retina and passed to the visual cortex, where a representation of the visual field is constructed.

Human rhodopsin has a quantum efficiency of about 25% (there's a 25% chance a single photon will be absorbed and produce the rod-cell signal). By comparison, cat rhodopsin is more than 90% quantum efficient. 25% QE is sufficiently high to be observable -- a source of single photons can be seen by a dark-adpated person with normal vision.

You'll need a steady source of well-collimated photons. A green laser pointer (~532 nm) will do nicely. But how many photons does it generate? A wavelength of 532 nm corresponds to 3.53×10-19 joules. So a small 1mW laser pointer puts out 2.8×1015 photons per second (watt = joule/s).

How do you reduce that to one photon at a time? With filters. An ND3 neutral density filter reduces the output light by 10-3 compared to the input, so a stack of just five ND3's in front of this laster pointer would result in (on average) just 2.8 photons per second! A stack of four ND4's would give you 0.28 photons/s on average.

If you don't have neutral density filters, you can make a decent approximation, by stacking sheets of black trash-bag plastic. To make this work you have to measure the attenuation yourself, so you'll need a photodetector, something which gives an output (voltage, resistance, current, whatever) proportional to the intensity of light.

Once you have your single-photon source, you need to set it up in a completely dark room. If you have access to an old-style photographic darkroom, use it. Otherwise, use thick (3-5 mm) black felt and gaffer's tape to seal any windows and doorframes. Put the laser on a table or stand pointed at your face, with the stack of NDs (or trash bags) in front of it. If you're doing this by yourself, you may want to have a piece of tape set up to hold the pointer's button down. Otherwise, your lab partner will take care of it.

Sit in the dark for 20 to 30 minutes. This will seem like forever, so you may want something to help you keep track of the time. A standard CD will be about half finished, or you can get through ten pop sons on your iPod, when your eyes become dark adapted.

Turn on the laser. You'll see intermittent flashes all coming from nearly the same place in your visual field; if you turn your head, the location will move in the opposite direction. If you've used filters to get down to a few flashes per second, POV will make them easier to see. At less than one photon per second, you'll see them individually.

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Goodhart8 years ago
And I just recently acquired a laser pointer....alas, it is not a green one. Can I still get this to work?
kelseymh (author)  Goodhart8 years ago
Definitely! Your eyes can still see all colors in the dark (consider the use of red filters for darkrooms, night-time flashlights, etc.). The green/blue range just gives you roughtly three times as many retinal cells to hit than the red.
8 years ago
I do have a filter that is very dark, but is it an infrared filter...that would filter out all BUT the infrared....something I don't want (although I might be able to see it on my digital camera ? )
kelseymh (author)  Goodhart8 years ago
Hmmm...that might work; the laser pointer is not putting out any IR, after all :-) If you have a spec sheet for the filter, see if it gives you any information on the residual transmittance in the visible. If it's below one per mil (0.1%), then you can use it just like the ND3 filter I mentioned above (you'll need four or five of them, of course :-)
7 years ago
I am still thinking about this.....but I am more focused of late on my trip to NYC  (HOPE conference)....I finally got my Hotel reservations in  *big grin*
8 years ago
I don't have any information on the filter (I got it from American Science and Supplus; and they are not very good at having spec sheets :-) I don't remember if I have more than one of them or not......I will have to find where I put it / them last :-) It is a pretty thick filter if I remember correctly. Some army surplus item...
artb607 years ago
how can one determine the physical size of a photon?
and If photons move through space in a helical manner, what causes them to do this?
I am really stumped. any thoughts?
kelseymh (author)  artb607 years ago
Following up on your first question (again), what do you mean by "physical size" of a photon?

-- Wavelength?  That's pretty obvious.

-- Spatial extent?  That depends on the time duration (equivalent to frequency spread) of the pulse.  If you're interested, I can go into detail.

-- Range of influence?  Since photons are massless, their range is infinite.  Not a particularly interesting answer :-)

kelseymh (author)  artb607 years ago
What do you mean by "physical size"?

Why do you think photons follow helical trajectories?  Could you give me a reference to where you got this idea?

Neither of these questions make any sense to me, so I can't give you any guidance toward an answer.  If you can help me get a clearer understanding of what you mean, maybe I can point you toward useful resources.
kelseymh (author) 7 years ago
Last year, I also put together an Instructable on how to make a double-slit interference setup.  It is also woefully lacking in pictures, but I've gone ahead and published it anyway.
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