How does income and substitution effects apply to the law of demand?
As the price of the commodity falls, the real income of the consumer increases. This induces the consumer to buy more of the same commodity. This is known as Income Effect.
The demand curve slopes downwards from left to right because of the substitution effect also. After a fall in the price of a good, its relative price also declines. It becomes relatively cheaper. The consumer will substitute this good in the place of other costly goods.