Problem — Rays for an object beyond 2f (lens)

Problem

Draw the ray diagram for a real object placed at a distance greater than twice the focal length ($p > 2f$) in front of a converging lens. State whether the image is real or virtual, upright or inverted, magnified or reduced.

Solution

Graphical construction. Two notable rays from the tip of the object are enough:

• the ray parallel to the optical axis, which after the lens passes through the focus $F'$;
• the ray that passes through the centre of the lens, which continues undeviated.

Their intersection (on the same side as the outgoing rays, hence real) locates the tip of the image.

Classification. For $p > 2f$ the image is:

$\ev{\text{real, inverted, reduced, located between } f \text{ and } 2f \text{ on the other side}}$

This is the regime of the camera: a distant object forms a small, inverted image just beyond the focus.

Collegamenti

Argomenti: Optics Concetti: Lenses and mirrors Competenze: Analysis of limiting cases and thought-experiment physics