Can we see single photons?

we are visual creatures and our eyes are marvelous at detecting light for us to see light enters our eyes passes through the lens then reaches the retina the retina contains cells called photoreceptors that can detect light and other cells transmit this information to the brain though we might not realize it when it comes to seeing in the dark our vision approaches or even reaches the fundamental physical limits of detection in this video let's learn about how our eyes work to answer the question how well can we see in the dark and is our night vision really that great [Music] photoreceptors are located at the back of the retina and consist of two types of cells cones and rods so named because of their shape cones allow us to see in bright light and in color as there are three types which are sensitive to red green and blue in dimmer light we rely on the rods as they are much more sensitive to light in this video we'll be focusing on rod cells as we rely on them for our night vision light is composed of discrete packets called photons in bright light we're bombarded by a deluge of photons like raindrops falling during a downpour a single rod or cone cell might receive millions of visible photons per second as the light becomes dimmer there are fewer photons until there are no visible photons in the dark we can only see visible photons that span the spectrum from red to purple we can't see all photons radio or infrared also are made of photons and are all around us but we're blind to them because our frauds and cones can't detect them for us to see in the dark our rods absorb and detect visible photons the physical limit for the dimmest light that a rod could possibly detect is a single photon a rod can't detect half a photon any more than a parent half half of a child or you could score half of a point in baseball is it possible for a broad cell to detect a single photon we could test this by measuring the dimmest flash of light that you can see this was done in the 1940s by three biologists named hex layer in parent reference in the description below first you need to dark adapt for about 45 minutes as you know when you enter a dark room everything seems dark at first over time your eyes adapt and you can see more easily because your pupils dilate your retina switches to using your rods for vision and the rods increase their sensitivity to light next they're straight ahead and flash a light off to the side when we look straight at something the light is focused onto a region of the retina that is almost all cones cones are great for seeing in bright light but not in the dark our peripheral vision about 20 degrees off to the side has the greatest density of rods and the best night vision you might have noticed this in the dark you might be able to see things in the periphery but when you stare straight at it it becomes harder to see so what is the dimmest light that most people can see a flash where the retina received only a single photon is too dim for anyone to perceive people can notice flashes where less than a dozen photons are absorbed and detected by the rods but there's a second important result we can notice a flash even if each of these dozen photons are received by different rods this means a rod cell can see a single photon but it takes several rods each receiving a photon for us to notice the ability of rods to detect single photons has been directly verified rods are filled with light sensing molecules called rhodopsin when a photon is absorbed by rhodopsin it activates the rhodopsin and ultimately produces a brief electrical signal that is passed on to the brain let's visualize this electrical signal in a dark adapted rod cell the absorption of a photon produces a small brief change in voltage in the rod i'll show the rod as blue while the electric signal is present the signal rapidly fades away so the rod can report the next photon that arrives these visualizations are slowed down to make it easier to see the real response up to a photon lasts less than a second a rod cell can detect individual photons and generate electric signals in response but it takes a brighter flash that activates many rods for us to notice why the reason is noise rhodopsin the molecule that detects photons occasionally makes a mistake and reports seeing a photon that wasn't there this happens a few times per minute per rod and results in the same electric signal that a visible photon would produce here we visualize this noise in complete darkness our brain has to rely on the electrical signals from rods as reporters of light with the many rods in the retina there's always some reporting seeing a photon even in complete darkness if there's a dim flash that results in a single rod detecting a photon there's no way the brain can distinguish that from the background noise but suppose there's a brighter flash detected by about a dozen rods it's unlikely that random chance will produce this many nearby rods all seeing photons at the same time while rods can detect single photons our brain requires corroboration from several rods simultaneously for us to notice the flash but you might have noticed that when you're completely dark adapted you might see a faint shimmering like tv static in your peripheral vision this is partially due to this noise so why not just make the rhodopsin better so there's less noise each rod contains about 100 million rhodopsin molecules each waiting to absorb a photon if there's a couple of noise events per rod per minute how often does a single rhodopsin molecule produce a noise event on average the answer works out to be once in 100 years rhodopsin is tremendously reliable compare that to the film used in old film cameras with an expiration date of about two years the film would go bad quicker if stored at body temperature which is how we store our rhodopsin cold-blooded animals such as frogs often have a lower body temperature than us and can have better night vision frog rods have lower background noise and they can see better in the dark than we can our vision isn't perfect though and there are several ways that things could be improved while rods can detect individual photons they don't detect every photon or rods usually detect less than 10 of the photons that reach the surface of the eye some of the photons are reflected off of the eye or pass through the retina without being absorbed nocturnal animals such as cats have a reflective layer behind the retina if a photon passes through the retina without being detected it bounces off the back and passes through again giving it a second chance to be seen also the color of the light matters our rods have difficulty detecting red photons and see blue-green photons best notice that rods and cones are on the back side of the retina and light passes through several cells before reaching the rods these cells can block some of the light it's kind of like having a camera with some wires and electronics between the lens and the sensor or film however all vertebrate eyes are arranged this way despite this our eyes are astonishingly sensitive the amount of energy carried by photons is minuscule carl sagan wrote in 1980 that the total amount of energy from outside the solar system ever received by all the radio telescopes on the planet earth is less than the energy of a single snowflake striking the ground for comparison how sensitive are our rods imagine you took all 8 billion people on the planet with normal eyesight and had them dark adapted show a blue flash of light just bright enough to be seen by all of these people the total amount of energy received by the rods of all the people on earth put together would be less than the energy of that single snowflake striking the ground you

Channel: Kerry Kim

Views: 11,109

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Id: BbhrHRsK1S4

Length: 7min 46sec (466 seconds)

Published: Tue Jan 19 2021