In a perfectly competitive market, the buyers treat products of all the firms as homogeneous.

A product being perfectly homogeneous implies that all units of a commodity are identical in size, quality, shape, colour, weight, etc. In a state of perfect competition, a perfectly homogeneous product is sold in the market at a uniform price. If ever an individual firm tries to charge higher price, it would lose all its buyers to a large number of other sellers in the market. In a perfectly competitive environment, homogeneous product does not allow a firm any control over its price. Accordingly, firm's demand curve (under perfect competition) becomes a horizontal straight line.