Why does the production possibilities frontier bow out Give

Why does the production possibilities frontier bow out? Give an example of two goods for which this would be the case.

Solution

The reason is that as more of one good is substituted for other good the opportunity cost increases. Given more resources it can shift outward. But given that there are limited number of resources the ppf tends to be concave because as more and more of one good is substituted for other good, the costs of such substitution increase. This can happen in production of coffee and tea.