Question

give reason - The electric bulb in a torch is kept at the focus of a concave mirror, WHY???

Answer 1

because concave lens diverge lights rays and show the light more bigger and glower

Answer 2

In a torch the bulb is placed at the focal point of the concave mirror so that the light from the bulb is reflected as a parallel or nearly parallel beam. This helps the user to see specifically what he wants to see with the help of the torch.

A street lamp uses a convex mirror so that the light from the bulb is spread as wide as possible. This allows the light to spread all over the street.