[ronit]

Elasticity of demand

Elasticity of demand is defined as percentage change in quantity demanded of a commodity to percentage change in any of the determinants of demand. Elasticity of demand explains quantitative relationship between quantity demanded of a commodity and its determinants. 
 
The degree by which quantity changes as price changes is called the price elasticity of demand. It is the percentage change in quantity to the percentage change in price (% Change in Quantity / % Change in Price). 

[ronit]

Perfectly Elastic :-Elasticity is infinite, when some 'small price reduction raises the demand from zero to infinity. Under such a case consumers will buy all that they can obtain of the commodity at some price. If there is a slight increase in price, they would not buy anything from the particular seller. This type of demand curve is found in perfectly competitive market.

Perfectly inelastic: Elasticity is zero, if there is no change at all in quantity demanded when price changes i.e. when quantity demanded does not respond to a price change.