How many photons does it take to see an object?

Chris Smith asked Physicist Michael Conterio to illuminate us on the subject...

Michael - So a photon is, we call it a particle of light. Because, in quantum physics, light is not just a wave it also can be divisible into these particular chunks of energy, and we call each one of those for light, a photon. You can’t have half a photon but what you can have is photons of different frequencies. So for different types of light it will have a different frequency, and a different amount of energy per photon. The cells in our retina are good at basically detecting particular types of photon, those in visible light. Not quite sure what you need in order to get a coherent image. It’s really hard to measure how many photons there are because they’re reacting with your eyes and, therefore, not going to set off whatever detector that we’re using.

Chris - Yeah. So, obviously, if you were to detect the photon you couldn’t, by definition, then expose your eye to it? You’ve got to sort of try and work out some way of saying well you know roughly how many photons must be arriving in the eye to trigger a response in the retina?

Michael - Yeah, precisely. But we think that when you’re getting down to the order of single digit numbers of photons, you can just about detect one or a few photons. It’s kind of a bit ambiguous there but you don’t recognisably see anything, it’s just like I think there was something there… maybe. But tens of photons you pretty much can and hundreds of photons, yes definitely.

Chris - What is the retina doing in order to enable you to detect these packets of energy (the photons) arriving on the retina?

Michael - When you get the photon arriving you’re basically transferring the energy from that photon to an electron in the retina, and that creates a signal that can then be interpreted by your brain as having seen some light. The thing is it’s not all photons that can do this. Some of them don’t have enough energy in order to actually bump that up.

We’re getting into chemistry here... you actually get some energies which the electron can’t jump up, so this is how your red, green, and blue cones in your retina which will each respond to a different, effectively, sized photon a different amount of energy in the photon. So some of them will just go straight past. Some of them, like ultraviolet light, you won’t detect as light; you won’t get the right sort of signal. But they can cause the electron to jump up a lot of energy and that will actually start causing damage in your eye because you’re now got a loose electron wondering about which can start doing damage to the cells in your eye.

On the other hand you have things like microwaves and radio waves, where the photons have lower energy and they just go straight through without interacting with it. There are lots and lots of photons going through your eye that you just never notice. So it’s kind of hard to say what’s the maximum number? It depends on the type of photon. UV ones will be causing damage; radio waves will just be going straight through.