In set theory the concept of empty set or null set is very important and interesting, it defined as the “a set which contains no elements is called as empty set or null set”, it is sometimes known as void set or vacuous set. It is usually denoted by this Greek symbol is known as phi.
This definition of empty seems to be opposing the definition of a set but this apparent contradiction when we understand the meanings of well defined objects. By well defined objects we mean that the objects which can be considered as the members of the set under consideration.
For example, a bottle is not well defined object for a jewelry set. Similarly, a chair is not well defined object for a tea set. But if we consider the set of necessary things of a house, then both of above mentioned things are well defined objects for such a set.
Thus if an object does not exist then it is a well defined object for an empty set.
For example,
 Odd numbers which are divisible by 2 are well defined objects for an empty set. A hen with horn is also we defined object for an empty set.

The positive integers which are less than 1 are also well defined objects for an empty set. If denotes the set of all positive integers, then the last set may be represented as

Let A be the set all those peoples whose heights are greater than 50 feet. According to the present statistical analysis of the world there is such person exists whose height is greater than 50 feet, so A is null set.

Let. In this situation B is empty set.