Meaning of Law of Demand:
                            Law of demand states that when other factors remain constant, quantity demanded changes due to change in price of the commodity i.e. when Price increases quantity demanded falls and vice-versa. There exists an inverse relationship between price and quantity demanded for a commodity. This law is also known as "First Law of Purchase". 

 Assumptions of Law of Demand :

 1. Tastes and preferences of a consumer do not change.

2. Income of consumer remains the same.

3. Price of substitute goods remain constant.

4. Price of complementary goods do not change.

5. There is no expectation of change in prices in future.

Explanation with Table and Diagram

Table shows that when price decreases, quantity demanded rises.

In Diagram, DD curve slopes downward showing inverse relationship between price and quantity demanded.