======Focal Length of a Convex Lens======


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**Alternative titles:** Convex Lens Focal Point
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====Summary====                   
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====Procedure====  
  - Set up the light box with a single slit to produce parallel rays of light.  
  - Place the convex lens in a lens holder on the optical table in the path of the rays.  
  - Position a viewing screen on the opposite side of the lens.  
  - Adjust the position of the lens or screen until the smallest and brightest spot of light appears.  
  - Measure the distance from the center of the lens to the screen; this is the focal length.



====Links==== 
Refraction of light through concave and convex lenses video - The Animated Teacher: 
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📄 Optics: Focal Length of a Lens - kbcc.cuny.edu: [[https://www.kbcc.cuny.edu/academicdepartments/physci/documents/phy_and_sci/SCI25_OpticsFocalLengthofaLens_Lab5.pdf]]\\

====Variations====
  * Try using lenses of different focal lengths and compare how far the screen must be placed to focus.  
  * Use a multi-slit light former to demonstrate how the lens converges several beams.  
  * Replace the light box with sunlight through a small window slit for a natural parallel beam.



====Safety Precautions====
  * Do not look directly into the light box beam.  
  * Handle lenses carefully to avoid scratches.  
  * Ensure electrical equipment (light box and power supply) is used safely and kept dry.




====Questions to Consider====
  * Why do convex lenses converge parallel rays to a focal point? (Because of refraction, light bends toward the principal axis as it passes through the curved surfaces.)  
  * How would the focal length change if the lens curvature increased? (A more curved lens would have a shorter focal length.)  
  * What is the difference between focal length and image distance? (Focal length is a fixed property of the lens, while image distance depends on where the object is placed.)