The demand curve slopes downwards due to:

The demand curve slopes downward due to I.E and S.E. According to S.E when a price of a commodity falls, it becomes relatively cheaper than other substitute commodities and as a result of this S.E, the quantity demanded of the commodity whose price has fallen, rises. According to I.E when the price of a commodity falls, the consumers real income or purchasing power increases, so he buy more of that commodity. This is called I.E.)