 In my last video I have been talking about radioactive isotopes and how they can undergo alpha, beta or gamma decay. Now one thing you might wonder is well how long does it go until one of these radioactive isotopes undergoes this decay? Like shouldn't they all just go away immediately? Now one single isotope it is very random when this decay is going to happen. However if you have many isotopes there is a way how we can statistically specify it and that way it's called the half-life. So what is a half-life? A half-life is the time it takes so that the amount of radioactive isotopes divides by two that's what we call it half. So after one half-life half of the isotopes have undergone the alpha, beta or gamma decay. So for example if I start with 100 grams of radioactive isotope after one half-life I will only have 50 grams left and the other 50 have transformed into another isotope through the decay and after another half-life and down to 25 grams and then to 12.5 and so on and so on and so on. So one half-life one half-life one half-life. Now how long is such a half-life? Well that depends on the isotopes. It can go from minutes hours to days to millions of years. One isotope you probably have heard of is C14. Its half-life is 5,730 years. So if I start with 100 grams of C14 after 5,730 years and down to 50 and to 25 and so on. So here this would be 100% 50% and 25% and so on 12.5%. Now what is interesting about C14 is how it gets generated. Like C14 gets generated in the higher atmosphere and it continuously gets regenerated from particles coming from the sun reacting with nitrogen in the higher atmosphere and it turns out in the atmosphere the amount of C14 remains more or less constant. So this is where it gets interesting. So let's say you have a tree and there's a lot. Now that tree will breathe in CO2 and out of the of the carbons that he breathes in with the CO2 and builds into itself there is going to be a fraction that is C14. Now as long as the tree is alive and keeps breathing that amount of C14 in the tree is always about the same as the one in the atmosphere which is more or less constant. Now the moment that the tree dies or gets cut the amount of C14 starts to go down according to the half-life jumps. So this is used in the C14 dating method. So what do we do in the C14 dating method? So you find a piece of wood you want to know how old it is. Let's say you found the building you found some paper from papyrus you want to know how old it is. What you're going to do is you're going to find out how much C14 is in the material you found and then you compare it to how much C14 is in a living organism of that type that you have just found. So let's say for example you found that your piece of wood that you want to date has only 25% of the C14 of a living tree and that would mean that your wood must have been cut down at least 5730 years then you get 50% and another 5730 years then you get to your 25% so then you could say okay my wood must have been cut down two times 5730 years so 11,460 years ago.