Describe as to how the age of fossils of plant and animals etc. is determined.

The method most commonly used, now, for estimating the age of fossils is radio-carbon dating. Radioactive isotope of carbon is commonly known as carbon- 14. Since carbon- 14 is chemically the same as ordinary carbon, both arc absorbed by plant and animal tissues in the same proportion as they arc present in the atmosphere as carbon dioxide. Plants use this carbon dioxide in making their food. Animals eat the plants. Hence, the proportion of carbon-14 in the tissues o f plants and animals is the same as in the atmosphere, as long as the plant or the animal is living. But as soon as it dies, no more carbon can enter its body as photosynthesis or food intake stops. Following death, the carbon-14, already present in the body, decays steadily into ordinary carbon. So the smaller the number of carbon-14 atoms remaining, the older is the fossil. This technique has been applied to materials of known age, and thus its accuracy was tested giving confidence in the age determinations of unknown samples. The carbon-14 method is applicable only to organic materials which still contain carbon. It cannot be used for fossils in which all organic matter has decayed. In that case the age of the fossil can be estimated by determining the presence of other radioactive elements like fluorine, or phosphorus.