Crosshairs on Image - Physics Demonstrations - Crosshairs on Image | Physics Undergraduate Labs

Purpose

Crosshairs positioned at the real image plane of a converging lens remain fixed relative to image features when viewed from different angles, proving the real image exists at a specific location in space. Addresses the common misconception that images only exist on screens.

Black ring with perpendicular threads forming crosshairs, mounted on ring stand with hanging weights for tension. Through the ring, the real inverted image of a colorful letter card (C visible) is seen superimposed on the crosshairs, with the convex lens on a stand visible behind.

Wide view of setup on green bench: crosshair ring on ring stand at left, convex lens on tabletop mount in center, wooden block with colorful letters A and C at right end of bench

Close-up of black ring with blue tape holding threads, perpendicular threads crossing at center, mounted on ring stand clamp with hanging weight for tension

Figure 1: Crosshairs at the real image plane. The inverted image of the letter card is visible through the ring, superimposed on the threads. When properly aligned, the threads stay fixed on the image features regardless of viewing angle.

Figure 2: Complete setup: object (letter cards on wooden block), 77 cm focal length convex lens on tabletop mount, and crosshair ring at the real image position.

Figure 3: Crosshair detail: threads are held taut by hanging masses and secured with blue tape. The threads cross at the center of the ring.