======Focal Length of a Concave Lens======


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**Alternative titles:** Concave Lens Focal Point
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====Summary====                   
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====Procedure====  
  - Place a light box with three parallel beams on an optical table or bench.  
  - Position a concave lens in a lens holder in the path of the rays.  
  - Observe the diverging rays after they pass through the lens.  
  - Extend the rays backward (using a screen with tracing paper or a ray diagram) until they meet at a virtual point.  
  - Measure the distance from the center of the lens to this intersection point; this is the principal focal length.



====Links==== 
Refraction of light through concave and convex lenses video - The Animated Teacher: 
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📄 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]]\\

====Variations====
  * 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.


====Safety Precautions====
  * 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.




====Questions to Consider====
  * 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.)