A small bulb is placed exactly at the principal focus of a convex lens. What kind of beam emerges from the other side of the lens?

1. By the lens rule, a ray passing through the principal focus emerges parallel to the principal axis. 2. A source placed at the focus sends rays that all leave parallel to the axis. 3. So a parallel beam emerges from the lens. 4. Convergence would require the source to be beyond the focus, so that option is wrong. _Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 2 Optics "Refraction through a Convex Lens", p.27_