Question

A small bulb is placed at the principal focus of a convex lens

Answer 1

Answer:

A small electric bulb is placed at the focus of a convex lens. It will produce a parallel beam of light and no image will be formed when the object is placed at the focus of a convex lens.

so,a small bulb is placed at the principal focus of a convex lens

Answer 2

Given that,

A small bulb is placed at the principal focus of a convex lens.

Convex lens :

If the focal  length of the lens is the positive. It means lens is convex lens.

If the small bulb is placed at the principal focus of convex lens then the light rays of bulb from focus will be reflected from the converging lens. It will produced parallel beam of light .

Hence, A small bulb is placed at the principal focus of a convex lens then the light ray will be parallel to principal axis.