Question

how is a water heater made with the help of a concave mirror

Answer 1

Concave mirror is known as the diverging mirror
Sunrays coming from infinity meet at a same point and focus on concave mirror
This produce a heat
Thus by this water heater made up of concave mirror

Hope this help you friend

Answer 2

concave mirror is reflected by sun rays they are converged ti the concave mirror and thus their intensity is increases . Thus water heater is made with the help with the help of a concave mirror and thus the water get heated through this procedure.