A concave lens causes parallel rays of light from a light box to diverge. By extending the diverging rays backward, their virtual intersection point is located, which defines the principal focal point. Measuring the distance from this point to the center of the lens gives the focal length.

1. Place a light box with three parallel beams on an optical table or bench.
2. Position a concave lens in a lens holder in the path of the rays.
3. Observe the diverging rays after they pass through the lens.
4. Extend the rays backward (using a screen with tracing paper or a ray diagram) until they meet at a virtual point.
5. Measure the distance from the center of the lens to this intersection point; this is the principal focal length.

Refraction of light through concave and convex lenses video - The Animated Teacher:

📄 Practical ways to find focal length – concave lens light box - Mammoth Memory: https://mammothmemory.net/physics/lenses/concave-lenses/practical-ways-to-find-focal-length--concave-lens-light-box.html

• Try concave lenses of different curvatures and compare their focal lengths.
• Replace the light box with sunlight through a narrow slit to demonstrate the same principle.

• Avoid looking directly into the bright light beams.
• Handle lenses carefully to prevent scratches.
• Keep the power supply and light box away from water or damp surfaces.

• Why does a concave lens cause light rays to diverge? (Because the curved surfaces bend light outward due to refraction.)
• Why is the focal point of a concave lens called “virtual”? (Because the rays do not actually meet but appear to originate from a point behind the lens.)
• How does increasing lens curvature affect the focal length? (A more curved lens has a shorter focal length because the rays diverge more strongly.)