A convex lens is used to converge parallel rays of light from a light box onto a screen. By adjusting the position of the screen until the sharpest focus is achieved, the distance from the lens to the focused spot gives the focal length of the lens.

1. Set up the light box with a single slit to produce parallel rays of light.
2. Place the convex lens in a lens holder on the optical table in the path of the rays.
3. Position a viewing screen on the opposite side of the lens.
4. Adjust the position of the lens or screen until the smallest and brightest spot of light appears.
5. Measure the distance from the center of the lens to the screen; this is the focal length.

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

📄 Optics: Focal Length of a Lens - kbcc.cuny.edu: https://www.kbcc.cuny.edu/academicdepartments/physci/documents/phy_and_sci/SCI25_OpticsFocalLengthofaLens_Lab5.pdf

• 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.

• 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.

• 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.)