Final answer:
In a perfectly competitive labour market, the labour supply curve is upward sloping, but when it comes to firms hiring, they face a horizontal supply curve for labour.
Step-by-step explanation:
In a perfectly competitive labour market, the labour supply curve is generally upward-sloping. This curve models the tradeoff between supplying labour into the market or opting for leisure activities, at every given wage level. As wages increase, more labour is willing to work, which means the quantity supplied of labour also increases, adhering to the law of supply. However, this scenario changes in a Perfectly Competitive Labour Market, where employers can hire all the workers, they want at the market wage. In such a market, firms face a horizontal supply curve for labour, indicating that an increase in wages does not affect the quantity of labour supplied, as it is already at full employment.