How many photons are there in a beam of light, and can we see a single one?
Each photon carries very little energy, so ordinary light holds an enormous number of them. A red laser pointer of 1 milliwatt sends out about 3 million billion photons every second. That is why light looks smooth and continuous: the individual packets are far too many to notice.
You dim the laser pointer with filters until it sends one photon per millionth of a second. By how much have you dimmed it? From 3.3 × 10^15 to 10^6 photons per second is a factor of about 3 × 10^9: three billion. The lab's first lesson does exactly this with a neutral density filter.
Our eyes are surprisingly good at this. The rod cells of the retina respond to a single photon, and in a famous 1942 experiment, people sitting in total darkness could report flashes made of only a handful of photons.
Special detectors do even better: they produce a click for each photon that hits them. With them, physicists can send light one photon at a time and watch what each one does. That is the starting point of quantum optics, and of the lab of this course.
Photons per second N = P / E = P λ / (h c), where P is the power in watts.
1 mW at 650 nm: N = 10^−3 / 3.06 × 10^−19 ≈ 3.3 × 10^15 photons per second, one every 0.3 femtoseconds on average.
A filter that lets through a fraction T of the light divides N by 1/T. Optical density OD = log10(1/T): a filter of OD 9 lets through one photon in a billion.
Ordinary light holds astronomical numbers of photons; dimmed enough, it arrives one photon at a time, and detectors can count every one.